fix: skip eval invocations without user content to prevent ValidationError

[tcconnally]

Problem

When a session contains invocations without user-authored events (author != "user"), convert_events_to_eval_invocations() creates an Invocation with an empty Content(parts=[]). This causes a Pydantic ValidationError because Invocation.user_content is typed as genai_types.Content and empty Content triggers validation failure.

Error when calling POST /apps/{app_name}/eval-sets/{eval_set_id}/add-session:

ValidationError on Invocation.user_content

Fix

Skip invocations entirely when no user event is found. Evaluations without user input are not meaningful, so filtering them out is both a correctness fix and an efficiency improvement.

Before

for invocation_id, events in events_by_invocation_id.items():
    user_content = Content(parts=[])
    ...
    invocations.append(Invocation(user_content=user_content, ...))

After

for invocation_id, events in events_by_invocation_id.items():
    user_content = Content(parts=[])
    ...
    if not user_content.parts:  # No user event found
        continue
    invocations.append(Invocation(user_content=user_content, ...))

Closes #3760