fix: respect A2A Message.role in inbound event conversion

[giulio-leone]

Summary

Fixes #5186.

convert_a2a_message_to_event() hard-coded role='model' on the GenAI Content it produced, ignoring the A2A Message.role field. This caused Role.user messages to be restored as model events.

Additionally, the task-history fallback in convert_a2a_task_to_event() (in event_converter.py) blindly took history[-1] without checking Message.role, so a trailing user message could be misattributed as agent output.

Root Cause

• event_converter.py: Content(role="model", ...) was hard-coded in two places inside convert_a2a_message_to_event().
• to_adk_event.py: _create_event() hard-coded role="model" with no way to override.
• event_converter.py: convert_a2a_task_to_event() fallback path used a2a_task.history[-1] unconditionally.

Changes

File	Change
converters/utils.py	New a2a_role_to_genai_role() helper (Role.agent→"model", Role.user→"user")
converters/event_converter.py	Use helper in convert_a2a_message_to_event(); filter history fallback to agent-role messages only
converters/to_adk_event.py	Add role param to _create_event(); pass mapped role from convert_a2a_message_to_event()
test_event_converter.py	Regression tests for role mapping + history fallback
test_to_adk.py	Regression tests for role mapping
test_utils.py	Unit tests for a2a_role_to_genai_role()

Testing

All 86 converter tests pass (test_event_converter.py, test_to_adk.py, test_utils.py).

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Response from ADK Triaging Agent

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Thank you!

[giulio-leone]

Added focused regression coverage for the fix:

• test_event_converter.py: verifies Role.user is restored as user and that task-history fallback ignores trailing user messages
• test_to_adk.py: verifies inbound event conversion preserves the mapped role
• test_utils.py: verifies a2a_role_to_genai_role() mappings

Local result: 86 converter tests passed (test_event_converter.py, test_to_adk.py, test_utils.py).

This reproduces the original bug path and confirms that user-role messages are no longer misattributed as model output.