docs(api): describe supported search rankers

[KXHXK]

What does this PR do?

Adds a schema description to SearchRankingOptions.ranker that lists the currently supported ranking algorithms: weighted, rrf, neural, and classifier.

The field remains str | None, so this is documentation-only and preserves acceptance of other strings for OpenAI API compatibility. A regression test verifies the generated Pydantic schema, and the checked-in OpenAPI specifications are regenerated.

Closes #6176

Test Plan

uv run ruff format --check src/ogx_api/vector_io/models.py tests/unit/providers/responses/builtin/test_openai_responses_file_search_ranking_options.py
uv run ruff check src/ogx_api/vector_io/models.py tests/unit/providers/responses/builtin/test_openai_responses_file_search_ranking_options.py
uv run pytest -q tests/unit/providers/responses/builtin/test_openai_responses_file_search_ranking_options.py
PYTHONUTF8=1 uv run ./scripts/run_openapi_generator.sh

Output:

2 files already formatted
All checks passed!
2 passed in 0.05s

Stable schema is valid
Experimental schema is valid
Deprecated schema is valid
Combined (stainless) schema is valid
OpenAPI specification generated successfully!
Schemas: 430
Paths: 50
Operations: 70

{
  "anyOf": [{"type": "string"}, {"type": "null"}],
  "default": null,
  "description": "Name of the ranking algorithm. Supported values are weighted, rrf, neural, and classifier. Other string values are accepted for OpenAI API compatibility but are not supported.",
  "title": "Ranker"
}

I also attempted the full pre-commit run twice; initialization could not fetch pre-commit-hooks because the GitHub connection timed out/reset. The relevant Ruff, pytest, schema validation, and generated-file checks above completed locally.

[mattf]

update the test to check that the description is present or things like "weighted" "rff" are in it

[KXHXK]

Thanks - updated the test as suggested. It now verifies that the schema description is present and independently checks for weighted, rrf, neural, and classifier, instead of coupling the test to the complete wording.

Verification on the updated test file:

• uv run pytest tests/unit/providers/responses/builtin/test_openai_responses_file_search_ranking_options.py -q - 2 passed
• uv run ruff check ... - passed
• uv run ruff format --check ... - passed

Current head: 89677f9.