fix: Fixes thought handling in contents.py and refactors its unit tests

Before this change, other agent's reply with thought will still be inserted in the outgoing LlmRequest due to the wrong `else` statement for calling all other type of part.

This commit also refactors test_contents.py to be behavior-oriented tests, instead of implementation-oriented, and add more test cases to cover expected scenarios.

The tests are divided into the following files with different focus:

- test_contents.py: covers the basic logic of event filter;
- test_contents_branch.py: covers the behavior related to branch, which takes effect when ParallelAgent is used.
- test_contents_other_agent.py: covers the retelling behavior to include other agents' reply as context for the current agent.
- test_contents_function.py: covers the function_call/function_response rearrangement logic mainly for `LongRunningFunctionTool`.

PiperOrigin-RevId: 802759821

tests/unittests/agents/test_remote_a2a_agent.py

@@ -515,7 +515,7 @@ class TestRemoteA2aAgentMessageHandling:
     self.mock_session.events = [mock_event]
 
     with patch(
-        "google.adk.agents.remote_a2a_agent._convert_foreign_event"
+        "google.adk.agents.remote_a2a_agent._present_other_agent_message"
     ) as mock_convert:
       mock_convert.return_value = mock_event
 
@@ -937,7 +937,7 @@ class TestRemoteA2aAgentIntegration:
 
     # Mock dependencies
     with patch(
-        "google.adk.agents.remote_a2a_agent._convert_foreign_event"
+        "google.adk.agents.remote_a2a_agent._present_other_agent_message"
     ) as mock_convert:
       mock_convert.return_value = mock_event