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 string "" (versions ≤ 1.19) or an empty Content(parts=[]) (current main).

Invocation.user_content is typed as a required genai_types.Content. Passing an empty string caused an immediate pydantic.ValidationError; 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, ...))  # may crash or be 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 raised ValidationError or produced incorrect data).
• 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.

🤖 Generated with Claude Code

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[adk-bot]

Response from ADK Triaging Agent

Hello @Koushik-Salammagari, thank you for your contribution!

It looks like the Contributor License Agreement (CLA) has not been signed yet. Could you please sign it? The PR cannot be merged without it.

Thanks!

[Koushik-Salammagari]

@googlebot I signed it!

[Koushik-Salammagari]

@googlebot I signed it!

[Koushik-Salammagari]

@googlebot I signed it!