fix(tools): hide skill script tool when no scripts exist

[Oxygen56]

Link to Issue or Description of Change

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

• Closes: SkillToolset exposes run_skill_script tool causing hallucination loop when skill has no scripts/ directory #6281

Problem:
SkillToolset always exposed run_skill_script, even when the configured local skills had no scripts. That gives the model an unavailable action surface and can contribute to repeated failed script calls.

Solution:
Only register RunSkillScriptTool when at least one local skill exposes scripts. Registry-backed toolsets keep the previous behavior because remote skill contents are not known at initialization time.

Testing Plan

Unit Tests:

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

Passed locally:

uv run --extra test pytest tests/unittests/tools/test_skill_toolset.py
# 115 passed, 4 warnings
uv run --extra dev ruff check src/google/adk/tools/skill_toolset.py tests/unittests/tools/test_skill_toolset.py
# All checks passed

Manual End-to-End (E2E) Tests:

Not run. The change is covered by isolated unit tests for the tool list returned by SkillToolset.

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 keeps the script execution tool out of the model-visible tool list when there is no local script to run, while preserving registry behavior for dynamically loaded remote skills.