A2A integration: Fix event conversion issues to align with A2A <-> ADK Go implementation

[kalenkevich]

Link to Issue or Description of Change

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

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

Problem:
The codebase required better separation of concerns and more robust handling for A2A Events. There was a need for improved conversion logic and accurate handling of event metadata, long-running tool IDs, and function call/response IDs as it previously relied on function names for tool matching (which is a bug!).

Solution:

• Extracted A2A Event definitions and builder functions into a new dedicated module core/src/a2a/a2a_event.ts.
• Updated core/src/a2a/event_converter_utils.ts to utilize the new event definitions and improved conversion logic between AdkEvent and A2AEvent (handling tasks, messages, and artifact updates).
• Enhanced core/src/a2a/part_converter_utils.ts to accurately handle id properties for functionCall and functionResponse instead of relying on function names for matching long-running tools.
• Updated metadata conversion utilities to safely handle missing optional fields.
• Added and updated comprehensive unit tests across a2a_event_test.ts, event_converter_utils_test.ts, part_converter_utils_test.ts, and metadata_converter_utils_test.ts to ensure full coverage and correct behavior.

Testing Plan

Unit Tests:

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

All unit tests in core/test/a2a/ pass successfully.

Manual End-to-End (E2E) Tests:

N/A - This is a refactoring and internal logic update covered by unit tests.

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

None

[kalenkevich]

Related to #189