test(tools): add unit tests for get_user_choice_tool

[Koushik-Salammagari]

Link to Issue or Description of Change

Description of the change (no existing issue):

Problem:
The google.adk.tools.get_user_choice_tool module had no dedicated unit-test
coverage.

Solution:
Add a focused, test-only module. No production code is changed.

Coverage added:

• get_user_choice — sets tool_context.actions.skip_summarization and
  returns None.
• get_user_choice_tool — is a LongRunningFunctionTool (is_long_running
  is True) wrapping the get_user_choice function, with name
  get_user_choice.

Testing Plan

Unit Tests:

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

$ pytest tests/unittests/tools/test_get_user_choice_tool.py -q
...
4 passed in 1.05s

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.