resolver: merge experimentalDecorators across tsconfig extends chain - #30478

PR #26436 added the experimental_decorators field on TSConfigJSON and
wired up the parse/consume sites, but forgot the merge step in
dirInfoUncached's extends-chain loop — emit_decorator_metadata is
OR-merged on the line just above it, experimental_decorators is not.
Any tsconfig that sets experimentalDecorators:true and also extends a
parent that doesn't set it silently loses the flag, so Bun emits
stage-3 decorators where legacy ones are expected (and reflect-metadata
throws).

Two drive-by fixes for --tsconfig-override while we're in here:

- transpiler.configureLinker now falls back to enclosing_tsconfig_json
  when the top-level directory has no tsconfig.json of its own. The
  override attaches to the root-directory DirInfo (parent == null),
  children only see it through the enclosing pointer, so decorator
  defaults were read from a null tsconfig.

- parseTSConfig for the override gets .invalid as its dirname_fd
  instead of the current directory's fd. The override path often lives
  somewhere that isn't a child of the dir we're iterating, which
  tripped the openat(dirname_fd, basename(path)) fallback in cache.zig
  and printed a bogus "Internal error: directory mismatch" warning.

Address review: the OR-merge made parent `true` sticky, so a child
tsconfig could not turn off `experimentalDecorators` or
`emitDecoratorMetadata` inherited from its base. TypeScript's
`extends` semantics are per-key override — a child's explicit value
wins, even when it's `false`.

Change the two fields on TSConfigJSON from `bool` to `?bool` so the
merge loop can distinguish "child didn't say" (null) from "child
explicitly set false". Consumers (resolver.dirInfoCached,
transpiler.configureLinker, JSTranspiler) unwrap with `orelse false`.

Two new test cases pin down the override semantics:
- child `experimentalDecorators: false` overrides parent `true`
  (base enables legacy decorators, child disables them → stage-3 emit).
- child `emitDecoratorMetadata: false` overrides parent `true`
  (bundler output must not contain `__legacyMetadataTS` calls).

- Tests 1-3 now assert `stderr === ""` so an unexpected resolver or
  transpiler warning on the non-override extends-chain paths fails the
  test instead of passing silently. Test 3 (override) keeps its
  targeted `not.toContain("directory mismatch")` — more specific than
  strict-equality for that regression.
- Bundler test asserts stdout before exitCode so a mismatch surfaces
  the actual build output in the failure diff.

…strict empty check)

Windows CI fails the strict `expect(stderr).toBe("")` — the test/
runtime on that platform emits harmless informational lines to stderr
that we don't want to gate the assertion on. Switch to the
conditional-on-failure pattern documented in test/CLAUDE.md: only
pin stderr when the exit code is non-zero, which still surfaces the
full stderr in the failure diff without making the green path
flaky.

…ts on 2019 x64 shard 3/8