Integrate openapi-schema-test-suite as canonical validator test suite

[Copilot]

Integrates the openapi-schema-test-suite — the canonical JSON test corpus for OAS 3.1/3.2 schema dialect validators — into the project's test suite.

Changes

Git submodule

• Added vendor/openapi-schema-test-suite pointing to the upstream repo

Test runner (tests/test_suite.py)

• Parametrizes all oas31/ and oas32/ JSON test files into 740 pytest cases with readable IDs (dialect/file/case/test)
• Enables format checkers for optional/format/ tests (format-as-assertion mode)
• 4 known failures marked xfail(strict=True):
  • format uri relative/invalid URI tests — uri format checker does not enforce RFC 3986 absolute-URI requirement

Bug fix: OAS 3.1/3.2 discriminator is annotation-only (validators.py)

The OAS 3.1 validator was inheriting custom oneOf/anyOf/allOf handlers that replace standard JSON Schema composition with discriminator-based $ref resolution (OAS 3.0 semantics). In OAS 3.1/3.2, discriminator is a pure annotation; composition validation is governed entirely by oneOf/anyOf/allOf. Removed those overrides so OAS 3.1/3.2 uses Draft 2020-12 semantics directly.

# Before: OAS31Validator with discriminator would silently attempt ref resolution
schema = {
    "$schema": "https://spec.openapis.org/oas/3.1/dialect/2024-11-10",
    "oneOf": [
        {"type": "object", "properties": {"petType": {"const": "cat"}, "meow": {"type": "string"}}, "required": ["petType", "meow"]},
    ],
    "discriminator": {"propertyName": "petType"},
}
v = OAS31Validator(schema)
list(v.iter_errors({"petType": "cat", "meow": "purr"}))
# Was: [ValidationError: "... reference '#/components/schemas/cat' could not be resolved"]
# Now: []  — discriminator ignored, oneOf validates normally

pyproject.toml

• Added vendor/openapi-schema-test-suite/tests to sdist includes

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