[1/n] Reduce the Bedrock fixture validation dependencies (#3654)

# Summary

This is part of a series to reduce the SDK’s dependency surface where a
small, purpose-built alternative is sufficient.

- Replace the general-purpose JSON Schema dependency with explicit
checks for our small, checked-in Bedrock authentication fixture.
- Keep the shared schema as the contract, and fail if it changes beyond
what the manual checks cover.
- Remove four development packages (`jsonschema`,
`jsonschema-specifications`, `referencing`, and `rpds-py`), reducing the
development dependency graph from 79 to 75 distributions. Runtime
dependencies are unchanged.

## Stack

- https://github.com/openai/openai-python/pull/3653 (merged)
- https://github.com/openai/openai-python/pull/3654 👈 this PR
- https://github.com/openai/openai-python/pull/3655
- https://github.com/openai/openai-python/pull/3656