test: add unit tests for CodeExecutionRequestProcessor and CodeExecutionResponseProcessor

[nuthalapativarun]

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Link to Issue or Description of Change

Adds unit tests for core/src/agents/processors/code_execution_request_processor.ts, which had no test coverage. The tests focus on the early-exit guard paths in both processors, verifying that they short-circuit correctly before reaching the complex code-execution logic that requires a live executor.

CodeExecutionRequestProcessor.runAsync — 3 tests:

• Non-LlmAgent agent → yields no events, leaves request unchanged
• LlmAgent with no codeExecutor → yields no events
• LlmAgent with a BaseCodeExecutor (non-built-in, optimizeDataFile = false) → runs runPreProcessor and exits cleanly; contents preserved

CodeExecutionResponseProcessor.runAsync — 5 tests:

• Partial response → yields no events (skips post-processing)
• Non-LlmAgent → yields no events
• LlmAgent with no codeExecutor → yields no events
• Response with no content → yields no events
• Response content with no code block → yields no events (no code to extract)

Testing Plan

Unit Tests:

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

✓ core/test/agents/processors/code_execution_request_processor_test.ts (8 tests) 6ms
Test Files: 1 passed (1)
     Tests: 8 passed (8)

Manual End-to-End (E2E) Tests:
Pure unit tests with no dependencies on external services.

Checklist

• Read CONTRIBUTING.md
• Self-reviewed code
• Commented hard-to-understand areas
• Added tests proving fix/feature works
• Unit tests pass locally
• Manually tested end-to-end
• Dependent changes merged/published

[kalenkevich]

@nuthalapativarun Hello, thanks for all your contributions. Can you please consolidate your docs and test PRs into 1?
Thanks!

[nuthalapativarun]

Closing in favour of the consolidated PR #413, as requested by @kalenkevich. All changes from this PR are included there.