fix(core): retry invalid structured model responses - #1433

Link to Issue or Description of Change

1. Link to an existing issue:

• Closes: SetModelResponseTool: schema validation failure terminates the tool flow instead of returning feedback #1430

Problem:

When set_model_response arguments fail output-schema validation, the validation exception prevents the model from receiving actionable feedback and retrying with a corrected structured response.

Solution:

• Return concise schema-validation feedback from set_model_response so the model can retry.
• Record successfully validated responses in EventActions.
• Promote only validated responses to the final structured model output.
• Remove the appended full schema from validation feedback to keep it concise.
• Add unit and Runner-level regression tests covering validation failure, feedback propagation, retry, and successful final output.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• The unit tests relevant to this change pass locally.

Tests executed:

• EventActionsTest
• OutputSchemaTest
• SetModelResponseToolTest

Result:

• BUILD SUCCESS
• 26 tests
• 0 failures
• 0 errors

Manual End-to-End (E2E) Tests:

Not run against a live model. A Runner-level regression test using the scripted TestLlm verifies that validation feedback from the invalid call is included in the retry request and that only the corrected, validated response becomes the final structured output.

Checklist

• I have read the CONTRIBUTING.md document.
• My pull request contains a single commit.
• 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.
• New and existing tests relevant to this change pass locally.
• I have manually tested the change end-to-end with a live model.
• No dependent downstream changes are required.

Additional context

This is my first open-source contribution and my first contribution to adk-java. I may have missed some repository conventions, so I would appreciate any guidance and am happy to make follow-up changes. Thank you for your time and review!