fix: namespace external-document pointers so same-named schemas don't conflate (#358)

[eseidel]

Summary

Fix multi-document schema-name collision (#358): two documents each
defining #/components/schemas/Foo used to conflate into one type. The
pipeline keys identity on JsonPointer, but an external OpenAPI-shaped
components library was parsed with a document-relative pointer, so
its Foo got the same #/components/schemas/Foo the root's Foo has.
Generation didn't fail — it silently emitted one foo.dart for two
types (whichever the walk reached first won).

JsonPointer gains an optional baseUri — the document it lives in —
that participates in equality but never in urlEncodedFragment.
Root pointers keep baseUri: null; the external components-library
parse threads baseUri: docUri. The two Foos become distinct
pointers → distinct types → distinct files, while the ref-resolvable
fragment (hence the registry key a $ref resolves against) is unchanged.

The root keeps the bare Foo; the external one disambiguates to Foo1
(foo_1.dart) — the same _1 convention an in-document name collision
already uses (Foo-Bar + Foo_Bar → FooBar + FooBar1), rather than
the name allocator's generic Foo2 fallback.

Details

• lib/src/types.dart — JsonPointer.fromParts({Uri? baseUri}), carried
  through add, added to props, left out of urlEncodedFragment.
• lib/src/parser.dart — ParseContext.baseUri threads through
  MapContext/ListContext child contexts into the pointer getter.
• lib/src/assemble.dart — the OpenAPI-shaped components-library parse
  sets baseUri: docUri. Split-spec targets already avoided this by
  keying on the full URI; this brings the components-library branch in
  line.
• lib/src/resolver.dart — _collectNames folds external-document
  schemas from the assembled registry into collision resolution, so a
  cross-document collision disambiguates through the same _1 path as an
  in-document one (root keeps the bare name; only external schemas are
  folded in, so the registry's copies of root schemas don't double-count).

Verification

• New unit tests on JsonPointer identity (baseUri disambiguates
  equality but not the emitted fragment) and add preservation.
• New end-to-end test: a root spec + external components library both
  defining Foo now emit Foo (localField, foo.dart) and Foo1
  (externalField, foo_1.dart) as distinct, analyze-clean types.
  Confirmed the test fails without the fix and passes with it.
• Byte-identical on every tracked fixture — tool/gen_tests.dart
  regens github/discord/backstage/petstore/spacetraders/train-travel
  with zero diff (single-document specs are all baseUri: null with no
  external schemas to fold in).

Known limitation (follow-up)

A discriminator mapping still rebuilds its match pointer from the ref
string (JsonPointer.parse, base-null), so a discriminator declared in
an external document remains a separate, pre-existing limitation
untouched here.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.69%. Comparing base (76adedc) to head (926695c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #364      +/-   ##
==========================================
+ Coverage   96.53%   96.69%   +0.15%     
==========================================
  Files          25       25              
  Lines        5888     5893       +5     
==========================================
+ Hits         5684     5698      +14     
+ Misses        204      195       -9     

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