Implements mock.module(specifier, options) on the node:test mock object,
matching Node 26.3.0. It replaces the module resolved from specifier with a
synthetic module exposing defaultExport and namedExports for the duration
of the test, working for both ESM import() and CJS require(), and returns a
MockModuleContext with .restore() that also participates in
mock.reset() / mock.restoreAll(). The legacy exports option is also
supported: its default key becomes the default export and its remaining own
enumerable keys become named exports; it cannot be combined with namedExports
or defaultExport.

The feature is built on top of module.registerHooks() (the module
customization hooks landed in #35026 / #35027) rather than a parallel
mechanism. The first mock.module() call lazily registers a single
resolve/load hook pair that intercepts both module systems:

• The load hook serves synthetic source for any resolved url that maps to an
  active mock. For a CommonJS module the named exports are applied onto the
  default export (throwing the same "Cannot create mock..." error Node throws
  when they cannot be); for an ESM module the default and named exports are
  exposed independently.
• The resolve hook versions the resolved url for ESM importers so that the
  default cache: false yields a fresh module namespace on every import while
  cache: true reuses a stable url. The CJS side is handled by evicting the
  require cache on setup and restoring the original cached instance on
  restore().

The live namedExports / defaultExport values cross into the synthetic
module through a registry published on globalThis under a well-known symbol;
the generated source reads them back by url key, so functions and objects pass
through unchanged.

The whole subsystem is primordial-only and lazily initialized: importing
node:test without mocking modules registers no hooks and adds no startup
cost, in line with the zero-cost-when-unused direction for the mock subsystem.

Spec tests under tests/specs/node/node_test_mock_module cover named + default
exports, the ESM importer, the CJS require importer, a .js module (ESM format
detection), non-identifier export names, the exports option and its
validations, the cache option, restore, and the interaction with
mock.reset() / mock.restoreAll().

The upstream Node conformance file parallel/test-runner-module-mocking.js is
intentionally not enabled in tests/node_compat/config.jsonc yet. The
node_compat runner judges a test file by its overall exit code, and that file
cannot pass as a whole in Deno for two independent reasons:

• It exercises features deferred from this PR: JSON mocking with
  with { type: "json" } import attributes (fix(ext/node): import attributes and custom module types in registerHooks ESM hooks #35028), automatic per-test mock
  restore (Deno's node:test has no per-test mock tracker yet), and live
  getter evaluation of defaultExport / exports.default (this PR snapshots
  export values eagerly).
• Some subtests spawn process.execPath with the Node-only flag
  --experimental-test-module-mocks, which Deno does not accept, so they fail
  regardless of feature completeness.

The implemented behavior is gated by the spec tests above; the conformance file
can be enabled once the deferred lifecycle / import-attribute work lands.

Deferred, and called out so they are not mistaken for regressions:

• JSON mocking and with { type: "json" } import attributes depend on the
  in-flight import-attributes / custom-module-type work (fix(ext/node): import attributes and custom module types in registerHooks ESM hooks #35028).
• Node restores each test context's mocks automatically when the test
  finishes; Deno's node:test has no per-test mock tracker yet, so the
  conformance cases that rely on automatic per-test restore still need that
  separate lifecycle feature. Explicit restore() / reset() /
  restoreAll() all work.
• defaultExport / exports.default getters are read once at mock setup
  rather than per module-system evaluation; live getter semantics need the
  registry to read through to the original options object.