Method probes that target a class or module under the Datadog namespace
let the tracer's own code paths re-enter the probe wrapper. The
re-entrancy guard added in #5560 prevents this for stdlib methods used
internally by DI; this commit takes the orthogonal approach for Datadog
namespace targets by rejecting them up front in hook_method.

A probe whose type_name equals "Datadog" or starts with "Datadog::"
now raises Error::ProbeTargetForbidden at hook time. The check is
textual on the probe's declared type name and runs before class
resolution, so non-existent Datadog-namespaced names are also rejected
with the new error rather than DITargetNotDefined. Line probes are
unaffected: they install via TracePoint, which is self-disabling during
its callback, and do not carry a type_name.

The error flows through ProbeManager#add_probe via the existing rescue
=> exc branch, so it is recorded as a failed probe and reported to the
backend as ERROR status without further changes.

Address review feedback on #5907:

- Strip Ruby's root-namespace prefix ("::") before the textual check,
  so a probe targeting "::Datadog::Tracing::SpanOperation" is rejected
  with ProbeTargetForbidden — same as the bare form. Without this,
  a "::"-prefixed name slipped past the check and fell through to
  Object.const_get, which raises NameError (rejected as
  DITargetNotDefined instead of ProbeTargetForbidden).

- Add tests for "::Datadog", "::Datadog::Tracing::SpanOperation",
  and "::DatadogLike" to cover both the rejection and the
  false-positive case under the normalization.

- Qualify the new RBS signature with the "::" root-namespace prefix
  for String, per the repo-wide RBS rule.

Verified: bundle exec rspec spec/datadog/di/instrumenter_spec.rb
(90 examples, 0 failures, 8 pending — same 8 Ruby-2-only pending
tests as before). bundle exec steep check lib/datadog/di/instrumenter.rb
clean.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

The textual rejection in hook_method missed alias forms that
Object.const_get still resolves to the same top-level constants. A method
probe with type_name "Object::Datadog::DI::Instrumenter" did not match
/\A(?:::)?Datadog(?:::|\z)/, but symbolize_class_name resolved it to the
real Datadog::DI::Instrumenter and prepended onto tracer internals —
exactly the re-entrancy/recursion hazard the rejection prevents. Top-level
constants are constants of Object, so any number of leading "Object::"
segments (with an optional leading "::") reaches the top-level Datadog.

Extend the regex to /\A(?:::)?(?:Object::)*Datadog(?:::|\z)/. "Object::" is
the only alias path: "Foo::Datadog" does not resolve through const_get for
an arbitrary Foo, and "Kernel::Datadog" raises NameError. The check stays
textual and pre-resolution, preserving the existing behavior that an
unknown Datadog-namespaced class raises ProbeTargetForbidden (not
DITargetNotDefined).

Specs add positive cases (Object::Datadog, Object::Datadog::DI::Instrumenter,
::Object::Datadog::Tracing::SpanOperation, Object::Object::Datadog) and a
negative case (Object::DatadogLike falls through to DITargetNotDefined).

Addresses codex review comment on PR #5907.

Verified: regex behavior checked directly against the full alias matrix via
ruby -e (all cases pass). instrumenter_spec.rb NOT executed: `bundle install`
fails in this environment (cannot resolve rake — no network access), so
`bundle exec rspec` cannot run. Spec syntax checked with `ruby -c`.