test: add more scenarios to test_end_to_end

[ishymko]

Based on https://a2a-protocol.org/latest/specification/#312-send-streaming-message:

1. Message based flow.
2. Emit Task as a first event.

TODO: switches to the old request handler as there are known issues in the new one

With a new handler failures are caused by

1. Task events are not streamed
2. return_immediately + direct message - V2 returns a phantom Task before the executor produces its Message

[github-actions]

🧪 Code Coverage (vs 1.0-dev)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/client/base_client.py	92.59%	95.56%	🟢 +2.96%
src/a2a/server/agent_execution/active_task.py	93.40%	94.06%	🟢 +0.66%
src/a2a/server/request_handlers/default_request_handler.py	97.82%	98.13%	🟢 +0.31%
src/a2a/server/tasks/task_manager.py	97.44%	98.29%	🟢 +0.85%
Total	92.23%	92.33%	🟢 +0.09%

Generated by coverage-comment.yml

[gemini-code-assist]

Code Review

This pull request refactors the MockAgentExecutor to support direct message responses without creating a task, and updates the task creation and status update logic. It introduces a LegacyRequestHandler and adjusts imports accordingly. Existing end-to-end tests were updated to align with the new task event structure and the removal of the explicit 'task submitted' message from the history. Additionally, new end-to-end tests were added to cover the direct message response functionality across different client configurations (blocking, streaming, and return_immediately). The review suggests combining the newly added direct message tests into a single parameterized test to improve maintainability and reduce code duplication, referencing a style guide rule about test class definition.