fix(tools): preserve const schemas for Gemini

[he-yufeng]

Summary

• accept JSON Schema const in the Gemini schema conversion path
• map const to a single-value enum before building the Gemini Schema, since the target type has no native const field
• add coverage for constant string schemas

Fixes #5653

To verify

• uv run --extra test python -m pytest tests\unittests\tools\test_gemini_schema_util.py -q
• uv run --extra dev pyink --check src\google\adk\tools\_gemini_schema_util.py tests\unittests\tools\test_gemini_schema_util.py
• uv run python -m py_compile src\google\adk\tools\_gemini_schema_util.py tests\unittests\tools\test_gemini_schema_util.py
• git diff --check

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