* refactor(express): trace middleware via layer prototype dispatch

Middleware tracing replaced `layer.handle` with a wrapper and re-exposed the
user handler through an enumerable `_datadog_orig` back-reference, the only
property loopback's `_findLayerByHandler` could follow to tag a layer with its
phase. That ties the tracer to loopback's private reflection heuristic, where
the property name and its enumerability are both load-bearing.

Wrap the host `Layer` prototype dispatch (`handle_request`/`handle_error`, and
`handleRequest`/`handleError` on router >=2) and read per-layer metadata from a
WeakMap, leaving `layer.handle` the user's function. loopback's first lookup
(`layer.handle === handler`) then matches with no tracer-specific contract, so
the back-reference, the `express-async-errors` `__handle` branch, and the
`_name` write-back onto the user handler are no longer needed. A synchronous
throw or a rejected promise becomes `next(error)` in the host dispatch, so the
wrapped `next` captures both without a tracer-side try/catch.

Refs: #9062

* fix(express): trace middleware on express <4.3.0 (no prototype dispatch)

express <4.3.0 has no `Layer.prototype.handle_request`/`handle_error`; the
router invokes `layer.handle` directly, so wrapping the prototype traced
nothing and dropped middleware, route-handler, and code-origin spans there.
Replace `layer.handle` in place (arity preserved so the host still routes error
handlers) when the layer exposes no dispatch method. express 4.3.0+, express 5,
the router package, and loopback keep `handle` pristine via the prototype wraps.

Also point the router-helper spec at the renamed `setLayerMeta` helper.

* test(router): cover express-async-errors on prototype-dispatch hosts

express-async-errors redefines `handle` as a getter/setter that stores the
wrapped handler in `__handle` and patches `handle` only, never `handle_request`.
On express 4.3.0+ the tracer's prototype-dispatch wrap therefore has to survive
that patch, the arity gate has to read the real 3-arg handler, and a rejected
async handler has to reach the wrapped `next` as `next(error)`. Pin that path so
a regression surfaces here instead of in a downstream tracer test.

* fix(express): restore error publishing for throwing middleware on express <4.6.0

On express <4.6.0 the router has no `Layer` prototype dispatch: it runs
`layer.handle` directly and, on a synchronous throw, catches outside the layer
and calls its own `next(error)` — never the wrapped `next` the tracer installed.
The prototype-dispatch rewrite dropped the legacy handle wrap's catch, so a
throwing middleware or route handler no longer published `middleware:error` /
`middleware:next` / `middleware:finish`: the span lost its error tag and lingered
on the stack until request finish.

Restore the catch in `wrapLegacyHandle` to publish error/next/finish and rethrow,
mirroring `wrapNext`. The prototype hosts (express >=4.6.0, express 5, router)
already route the throw through the wrapped next, so only the legacy path needs it.

Also correct the version boundary in the surrounding comments: `handle_request`
landed in express 4.6.0, not 4.3.0, so the legacy path covers `>=4.0.0 <4.6.0`.

Refs: #9067 (comment)

* fix(express): publish middleware next/finish once per continuation

A handler that calls `next()` and then rejects (`async (req, res, next) => {
next(); await bg() }` where `bg()` rejects) makes the host invoke the same
wrapped `next` twice: the clean `next()` finishes and pops the middleware span,
then the host's rejection pass calls `next(error)` again. Since the middleware
stack is already empty, that second pass tagged the error on the parent span,
inventing a request-level error the middleware never surfaced.

Guard the wrapped continuation so its error/next/finish publishes fire at most
once per dispatch; the second call still forwards to the host chain untouched.

Add the missing JSDoc on `hasLayerDispatch`.

* refactor(express): trace middleware via layer prototype dispatch

Middleware tracing replaced `layer.handle` with a wrapper and re-exposed the
user handler through an enumerable `_datadog_orig` back-reference, the only
property loopback's `_findLayerByHandler` could follow to tag a layer with its
phase. That ties the tracer to loopback's private reflection heuristic, where
the property name and its enumerability are both load-bearing.

Wrap the host `Layer` prototype dispatch (`handle_request`/`handle_error`, and
`handleRequest`/`handleError` on router >=2) and read per-layer metadata from a
WeakMap, leaving `layer.handle` the user's function. loopback's first lookup
(`layer.handle === handler`) then matches with no tracer-specific contract, so
the back-reference, the `express-async-errors` `__handle` branch, and the
`_name` write-back onto the user handler are no longer needed. A synchronous
throw or a rejected promise becomes `next(error)` in the host dispatch, so the
wrapped `next` captures both without a tracer-side try/catch.

Refs: #9062

* fix(express): trace middleware on express <4.3.0 (no prototype dispatch)

express <4.3.0 has no `Layer.prototype.handle_request`/`handle_error`; the
router invokes `layer.handle` directly, so wrapping the prototype traced
nothing and dropped middleware, route-handler, and code-origin spans there.
Replace `layer.handle` in place (arity preserved so the host still routes error
handlers) when the layer exposes no dispatch method. express 4.3.0+, express 5,
the router package, and loopback keep `handle` pristine via the prototype wraps.

Also point the router-helper spec at the renamed `setLayerMeta` helper.

* test(router): cover express-async-errors on prototype-dispatch hosts

express-async-errors redefines `handle` as a getter/setter that stores the
wrapped handler in `__handle` and patches `handle` only, never `handle_request`.
On express 4.3.0+ the tracer's prototype-dispatch wrap therefore has to survive
that patch, the arity gate has to read the real 3-arg handler, and a rejected
async handler has to reach the wrapped `next` as `next(error)`. Pin that path so
a regression surfaces here instead of in a downstream tracer test.

* fix(express): restore error publishing for throwing middleware on express <4.6.0

On express <4.6.0 the router has no `Layer` prototype dispatch: it runs
`layer.handle` directly and, on a synchronous throw, catches outside the layer
and calls its own `next(error)` — never the wrapped `next` the tracer installed.
The prototype-dispatch rewrite dropped the legacy handle wrap's catch, so a
throwing middleware or route handler no longer published `middleware:error` /
`middleware:next` / `middleware:finish`: the span lost its error tag and lingered
on the stack until request finish.

Restore the catch in `wrapLegacyHandle` to publish error/next/finish and rethrow,
mirroring `wrapNext`. The prototype hosts (express >=4.6.0, express 5, router)
already route the throw through the wrapped next, so only the legacy path needs it.

Also correct the version boundary in the surrounding comments: `handle_request`
landed in express 4.6.0, not 4.3.0, so the legacy path covers `>=4.0.0 <4.6.0`.

Refs: #9067 (comment)

* fix(express): publish middleware next/finish once per continuation

A handler that calls `next()` and then rejects (`async (req, res, next) => {
next(); await bg() }` where `bg()` rejects) makes the host invoke the same
wrapped `next` twice: the clean `next()` finishes and pops the middleware span,
then the host's rejection pass calls `next(error)` again. Since the middleware
stack is already empty, that second pass tagged the error on the parent span,
inventing a request-level error the middleware never surfaced.

Guard the wrapped continuation so its error/next/finish publishes fire at most
once per dispatch; the second call still forwards to the host chain untouched.

Add the missing JSDoc on `hasLayerDispatch`.