Expose the Gemini Live API client

[sylvesterkaczmarek]

Closes #5862

Summary

• Exposes Gemini.live_api_client as the documented public override point for Live API connections.
• Keeps _live_api_client as a compatibility alias.
• Preserves subclasses that already override the former private property.
• Updates connect() to use the public property.
• Adds tests for the public override, compatibility alias and legacy override path.

Validation

• pytest tests/unittests/models/test_google_llm.py -q → 92 passed
• Pre-commit checks on all changed files → passed

A remote Live API endpoint was not used. The unit suite covers client creation and connection setup.