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!

We noticed a few things to align with our contribution guidelines:

• Contributor License Agreement (CLA): It seems the Google CLA check has failed. Please make sure to sign the Contributor License Agreement at https://cla.developers.google.com/ to proceed.
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These steps will help the reviewers better understand and review your changes efficiently. Thanks for your contribution!

[rohityan]

Hi @ynachiket , Thank you for your contribution! It appears you haven't yet signed the Contributor License Agreement (CLA). Please visit https://cla.developers.google.com/ to complete the signing process. Once the CLA is signed, we'll be able to proceed with the review of your PR. Thank you!