fix: Support pipe operator (X | Y) union syntax in function parameter parser

[AhrendsW]

Summary

• Fixes Error adk_web_server.py:1431 - Error in event_generator: Failed to parse the parameter file_patterns: List[str] | None = None of function cleanup_unused_files #3591 — The function parameter parser did not recognize types.UnionType (created by Python 3.10+ pipe syntax like list[str] | None), only typing.Union. This caused a ValueError and fallback to Pydantic, which didn't always work (e.g., in the web builder).
• Adds types.UnionType checks in three conditions within _parse_schema_from_parameter() in _function_parameter_parse_util.py:
  1. Simple union handler (get_origin check)
  2. Outer isinstance guard for the GenericAlias block
  3. Inner union check inside the GenericAlias block
• Adds 8 new tests covering pipe union variants (optional list/dict, defaults, simple primitives, complex multi-type, GEMINI_API fallback, and typing.Union regression)

Test plan

• All 43 tests in test_build_function_declaration.py pass
• Full unittest suite passes (4017 passed, 0 failures)
• pyink and isort formatting verified via autoformat.sh
• Lint checks pass (pyink --check reports no changes needed)
• No merge conflicts with main

[gemini-code-assist]

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