test(events): add unit tests for Event helper methods

[Koushik-Salammagari]

Link to Issue or Description of Change

Or, if no issue exists, describe the change:

Problem:
The Event class's helper methods — is_final_response(), get_function_calls(), get_function_responses(), has_trailing_code_execution_result(), and automatic id generation in model_post_init — had no dedicated unit tests. They were only exercised incidentally through higher-level tests, and there was no tests/unittests/events/ package at all. This left several branches (early-return overrides, empty-content edge cases) without direct coverage.

Solution:
Add tests/unittests/events/test_event.py with focused, isolated unit tests for these methods, including edge cases:

• is_final_response() — plain/empty events are final; function-call/response, partial, and trailing code-execution results are not final; skip_summarization and long_running_tool_ids early-return overrides.
• get_function_calls() / get_function_responses() — ordering, no-content, empty-parts, and text-only cases.
• has_trailing_code_execution_result() — last vs. non-last part, no-content, and empty-parts cases.
• id auto-generation — auto-assigned when missing, unique across events, and preserved when explicitly provided.

These are pure-logic tests with no external dependencies; no runtime behavior is changed.

Testing Plan

Unit Tests:

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

$ pytest tests/unittests/events/test_event.py -q
22 passed, 5 warnings in 1.02s

Manual End-to-End (E2E) Tests:

N/A — this is a test-only change and does not modify any runtime behavior.

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.
• Any dependent changes have been merged and published in downstream modules.