fix(evaluation): skip invocations without user events to prevent ValidationError

[Koushik-Salammagari]

Link to Issue or Description of Change

Closes #3760

Description

EvaluationGenerator.convert_events_to_eval_invocations iterates over events grouped by invocation_id. For invocations that contain no user-authored event (e.g. internal background turns, system-driven tasks where all events have author != "user"), the code previously left user_content as an empty Content(parts=[]).

Invocation.user_content is typed as a required genai_types.Content. Passing Content(parts=[]) silently creates a semantically incorrect eval case — an invocation with no user turn is not meaningful for evaluation.

The fix: initialise user_content = None and skip the invocation if no user event is found. A DEBUG log line is emitted so operators can diagnose unexpected skips without producing noisy warnings.

# Before
user_content = Content(parts=[])   # always appended, even with no user event
...
invocations.append(Invocation(user_content=user_content, ...))  # semantically wrong

# After
user_content = None
...
if user_content is None:
    logger.debug('Skipping invocation %s: no user-authored event found.', invocation_id)
    continue
invocations.append(Invocation(user_content=user_content, ...))

Changes

• src/google/adk/evaluation/evaluation_generator.py: initialise user_content = None; skip invocations where no user event was found.
• tests/unittests/evaluation/test_evaluation_generator.py: add two regression tests.

Testing Plan

• test_invocation_without_user_event_is_skipped — single agent-only invocation returns empty list (previously produced an incorrect eval case).
• test_mixed_invocations_skips_only_agent_only_ones — verifies that valid user+agent invocations are retained and only agent-only ones are filtered out.
• All 9 TestConvertEventsToEvalInvocation tests pass.

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[Koushik-Salammagari]

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[rohityan]

Hi @Koushik-Salammagari , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Please fix formatting errors by running autoformat.sh

[Koushik-Salammagari]

Hi @rohityan — formatting has been fixed via autoformat.sh and all CI checks are green. Ready for re-review whenever you get a chance!