test(tools): add unit tests for ToolConfirmation

[Koushik-Salammagari]

Link to Issue or Description of Change

Description of the change (no existing issue):

Problem:
The ToolConfirmation model in google.adk.tools.tool_confirmation had no
dedicated unit-test coverage.

Solution:
Add a focused, test-only module. No production code is changed. The model is
gated behind an experimental feature flag (which emits a UserWarning on use);
the tests filter that expected warning.

Coverage added:

• Default values (hint="", confirmed=False, payload=None).
• Custom values are stored as provided.
• payload accepts arbitrary JSON-serializable values (lists, strings).
• extra="forbid" rejects unknown fields with a ValidationError.
• The model round-trips through model_dump/model_validate.

Testing Plan

Unit Tests:

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

$ pytest tests/unittests/tools/test_tool_confirmation.py -q
...
5 passed in 0.96s

Manual End-to-End (E2E) Tests:

Not applicable — this is a test-only change with no runtime/user-facing impact.

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.