refactor: simplify GTFS entity container parameterization

[mackenziereading19]

Summary

Simplifies GtfsEntityContainer by removing the unnecessary descriptor type parameter.

GtfsEntityContainer<T, D> stored the descriptor as D, but callers only rely on the base GtfsFileDescriptor API. This change reduces it to GtfsEntityContainer<T> while preserving the descriptor-specific generic on GtfsTableContainer<T, D>, where it is still useful.

Fixes #1865.

Changes

• remove D extends GtfsFileDescriptor from GtfsEntityContainer;
• store and expose the descriptor as GtfsFileDescriptor;
• update affected container, loader, validator-provider, report-summary, GeoJSON, and translation references;
• preserve GtfsTableContainer<T, D> and its table-descriptor typing.

Validation

• :core:compileJava — pass
• :main:compileJava — pass
• :core:test — pass
• targeted FeedMetadataTest — pass
• targeted TranslationFieldAndReferenceValidatorTest — pass
• :core:spotlessCheck :main:spotlessCheck — pass
• git diff --check — pass
• no remaining two-argument GtfsEntityContainer<..., ...> references

Full :main:test completed 678 tests with 3 failures. The failures are the existing Mockito initialization failures in:

• JsonReportSummaryGeneratorTest.withFeedMetadataWithConfigTest
• JsonReportSummaryGeneratorTest.withFeedMetadataNoConfigTest
• VersionResolverTest

These failures reproduce on the untouched baseline and are unrelated to this refactor.