LCORE-3574: Added custom serializer

[asimurka]

Description

Adds dump_ogx_model() via ApiClient.sanitize_for_serialization() so OGX responses and streaming chunks serialize correctly through nested OneOf fields that plain model_dump() leaves empty. The responses, prompts, and providers endpoints now use this helper instead of calling model_dump() directly on ogx_client models.

Integration tests build real OpenAIResponseObject instances instead of mocks. Unit tests patch dump_ogx_model with explicit serialized payloads where mocks are still used. Serialization behavior is covered by new tests in test_ogx_serialization.py.

Type of change

• Refactor
• New feature
• Bug fix
• CVE fix
• Optimization
• Documentation Update
• Configuration Update
• Bump-up service version
• Bump-up dependent library [pyproject.toml + uv.lock]
• Bump-up dependent library [requirements.*.txt for Konflux]
• Bump-up library or tool used for development (does not change the final image)
• CI configuration change
• Konflux configuration change
• Unit tests improvement
• Integration tests improvement
• End to end tests improvement
• Benchmarks improvement

Tools used to create PR

Identify any AI code assistants used in this PR (for transparency and review context)

• Assisted-by: Cursor

Related Tickets & Documents

• Related Issue #
• Closes # https://redhat.atlassian.net/browse/LCORE-3574

Checklist before requesting a review

• I have performed a self-review of my code.
• PR has passed all pre-merge test jobs.
• If it is a core feature, I have added thorough tests.

Testing

• Please provide detailed steps to perform tests related to this code change.
• How were the fix/results from this change verified? Please provide relevant screenshots or results.

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[tisnik]

LGTM