feat: add random choice default tool

[lntutor]

Summary

Adds the llm_random_choice default tool proposed in #1178. The tool accepts a list of strings and returns one entry using random.choice, and is registered alongside llm_version and llm_time.

The regression test patches the chooser deterministically and invokes the tool through the real llm -T CLI path, covering registration, argument decoding, and the returned tool result.

Fixes #1178.

Testing

• uv run pytest tests/test_tools.py (35 passed)
• uv run ruff check llm/tools.py llm/default_plugins/default_tools.py tests/test_tools.py
• uv run black --check llm/tools.py llm/default_plugins/default_tools.py tests/test_tools.py
• git diff --check