Stop interpreting x-nullable

[ChiragAgg5k]

Summary

• stop treating the non-standard x-nullable extension as schema nullability
• exclude it when lifting OpenAPI 2.0 parameter fields into schemas
• preserve it only as an uninterpreted schema extension and update cross-version fixtures

Validation

• composer check

[greptile-apps]

Greptile Summary

The PR stops interpreting the vendor-specific x-nullable extension as canonical schema nullability.

• Removes x-nullable from OpenAPI 2.0 parameter fields lifted into synthesized schemas.
• Preserves schema-level x-nullable as opaque extension metadata.
• Updates unit tests and cross-version fixtures to reflect the revised behavior.

Confidence Score: 5/5

The PR appears safe to merge with no concrete blocking or independently actionable non-blocking issues identified.

The parser and tests consistently implement the stated opaque-extension contract while retaining dedicated coverage for standard OpenAPI 3.0 and 3.1 nullability.

Important Files Changed

Filename	Overview
src/Parser/Schema/Reader.php	Removes the x-nullable fallback from schema nullability and excludes the extension from OpenAPI 2.0 parameter-field lifting, consistently implementing the stated behavior.
tests/Schema/ReaderTest.php	Verifies that standard nullable remains supported, schema-level x-nullable stays opaque, and parameter lifting excludes it.
tests/CrossVersionFixtureTest.php	Updates canonical cross-version expectations so the fixture nickname is non-nullable in all dialects.
tests/Fixtures/openapi-2.0.json	Removes the non-standard nullability extension from the equivalent OpenAPI 2.0 fixture.
tests/Fixtures/openapi-3.0.json	Removes standard nullability from the OpenAPI 3.0 fixture to preserve cross-version semantic equivalence.
tests/Fixtures/openapi-3.1.json	Removes the null type from the OpenAPI 3.1 fixture to preserve cross-version semantic equivalence.

_{Reviews (1): Last reviewed commit: "fix: stop interpreting x-nullable" | Re-trigger Greptile}