fix(evals): skip invocations without user events

[ynachiket]

Summary

• Skip grouped eval invocations that never include a user-authored event.
• Keep normal user/agent invocation conversion unchanged.
• Add regression coverage for sessions containing an agent-only invocation followed by a valid user turn.

Fixes #3760.

Validation

• /tmp/adk-pdlc051-venv/bin/python -m pytest tests/unittests/evaluation/test_evaluation_generator.py -q (19 passed, 4 warnings)
• /tmp/adk-pdlc051-venv/bin/python -m py_compile src/google/adk/evaluation/evaluation_generator.py tests/unittests/evaluation/test_evaluation_generator.py
• git diff --check

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[adk-bot]

Response from ADK Triaging Agent

Hello @ynachiket, thank you for your pull request!

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