Drop leftover TrajectoryDiagnosticsResult reference

Commit acc1404 removed the TrajectoryDiagnosticsResult class but left
it inside the VerificationResultEvent discriminated Union, which made
`from webeval.rubric_agent.data_point import *` raise NameError at
import time and broke every test that transitively imports the module
(21 failures / errors across test_rubric_agent_imports,
test_shared_data_adapter, test_webtailbench_dataset,
test_verify_trajectories). Removing the orphan reference restores the
test suite to 27 passed / 2 skipped.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

webeval/src/webeval/rubric_agent/data_point.py

@@ -557,7 +557,6 @@ VerificationResultEvent = Annotated[
         MMRubricResult,
         MMRubricOutcomeResult,
         WebJudgeResult,
-        TrajectoryDiagnosticsResult,
         TaskAgentResult,
         VerificationResult,
     ],