fix(eval): ignore UI-only InvocationEvent fields on eval case save

[Anusha0501]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: 422 Unprocessable Entity in versions 2.6.0, 2.7.0, 2.8.0 when trying to save an eval case #7019
• Related: fix(eval): default RubricContent.text_property to None so rubrics save without 422 #7026 (different 422: missing RubricContent.text_property default; does not cover this extra-fields case)

2. Or, if no issue exists, describe the change:

Problem:
Saving an eval case from the adk web editor returns HTTP 422 Unprocessable Entity. The editor serializes UI-only transcript fields (invocationIndex, toolUseIndex) onto each InvocationEvent. EvalBaseModel uses extra="forbid", so FastAPI rejects the PUT body as extra_forbidden.

Solution:
Override InvocationEvent with extra="ignore" (same ConfigDict pattern as EvalCase / SessionInput). Unknown UI fields are dropped at parse time, the save succeeds, and those indices are not persisted in the eval-case JSON.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

PYTHONPATH=src pytest tests/unittests/evaluation/test_eval_case.py

23 passed

Added test_eval_case_put_accepts_web_ui_transcript_indices, which validates the reporter payload from #7019 and asserts invocationIndex / toolUseIndex are not stored.

Manual End-to-End (E2E) Tests:

• Run adk web for an agent that uses tools
• Create a session, add it to an eval set, edit a message, click Save
• Expect HTTP 200 (not 422)
• Confirm the stored eval-case JSON does not contain invocationIndex or toolUseIndex

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

#7026 only defaults RubricContent.text_property. The reporter confirmed that does not fix this save. Maintainer diagnosis on #7019: extra_forbidden on invocationIndex and toolUseIndex inside conversation[0].intermediateData.invocationEvents.

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