fix(tools): handle tuple schemas in function parsing fallback

[kaligautier]

Please ensure you have read the contribution guide before creating a pull request.

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: FunctionTool automatic parsing fails on list[tuple[bytes, str]] parameter type #5428
• Related: N/A

2. Or, if no issue exists, describe the change:

N/A (issue exists).

Problem:
FunctionTool automatic declaration parsing crashes when a parameter uses
list[tuple[bytes, str]] | None. The fallback path relies on Pydantic schema
output, but google.genai.types.Schema rejects tuple prefixItems, causing a
startup failure.

Solution:
Normalize tuple-related JSON Schema output in the fallback parser before
validation by types.Schema:

• convert prefixItems to a supported shape (items.anyOf)
• drop unsupported tuple-only fields (unevaluatedItems, items: false)
• add a regression test that covers list[tuple[bytes, str]] | None

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Passed locally:

• uv run --with pytest --with pytest-asyncio pytest tests/unittests/tools/test_from_function_with_options.py -q → 17 passed
• uv run --with pytest --with pytest-asyncio pytest tests/unittests/tools/test_build_function_declaration.py tests/unittests/tools/test_function_tool.py tests/unittests/tools/test_from_function_with_options.py -q → 83 passed

Manual End-to-End (E2E) Tests:

Manual validation using the issue repro signature:

• Ran from_function_with_options with input_bytes: list[tuple[bytes, str]] | None
• Verified declaration is generated (no startup crash), required fields are
  computed correctly, and the optional tuple parameter remains nullable.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

This fix intentionally targets schema compatibility in the legacy automatic
function declaration fallback path without changing tool behavior.