fix(a2a): preserve custom client factories

[gioboa]

Link to Issue or Description of Change

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

• Closes: RemoteA2aAgent resets A2AClientFactory #3158

Problem:

RemoteA2aAgent rebuilt a caller-provided A2AClientFactory subclass as the base factory when creating its HTTP client. This discarded overridden behaviour such as a custom create() implementation.

Solution:

Only rebuild the standard A2AClientFactory. Preserve subclasses so their custom client creation behavior remains active.

Testing Plan

Unit Tests:

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

uv run pytest tests/unittests/agents/test_remote_a2a_agent.py -q
231 passed

uv run pytest tests/unittests/a2a/test_compat.py -q
22 passed, 25 skipped

The regression test provides a factory subclass with an overridden create() method and verifies that it creates the remote client.

Manual End-to-End (E2E) Tests:

1. Started a local A2A server with a deterministic response.
2. Created a custom factory without an HTTP client, reproducing the original replacement path.
3. Ran a RemoteA2aAgent through a real Runner.
4. Verified the custom factory created the client, the server received the request, and ADK returned the response.

factory_create_calls=1
server_requests=1
response_received=True
E2E PASS

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.