fix: include delimiter when matching events from parent nodes in content processor

Previously we only do a simple prefix string matching, thus `agent_00` will match with `agent_0`

With this new change, we either check directly equality, or must expect seeing `agent_0.`. See added test for branches we now match / skip.

TBF `.` is also not a perfect delimiter (I would imagine users might put dot in agent names). We might consider a follow up that bans such agent names.

Tested with script in the linked issue (I updated prompt so we see which agent they see from):

Before:

```
[agent_8]: 73
[agent_0]: 97
[agent_1]: 73
[agent_5]: 97
[agent_4]: 73
[agent_2]: 73
[agent_3]: 73
[agent_9]: 93
[agent_6]: 73
[agent_7]: 1
[agent_70]: 1 (agent_7)
[agent_20]: 73 (agent_2)
[agent_30]: 73 (agent_3)
[agent_00]: 97 (agent_0)
[agent_40]: 73 (agent_4)
[agent_80]: 73 (agent_8)
[agent_50]: 97 (agent_5)
[agent_90]: 93 (agent_9)
[agent_10]: 73 (agent_1)
[agent_60]: 73 (agent_6)
```

After:
```
[agent_9]: 73
[agent_6]: 73
[agent_2]: 73
[agent_7]: 93
[agent_4]: 73
[agent_1]: 73
[agent_3]: 73
[agent_5]: 97
[agent_0]: 73
[agent_8]: 87
[agent_50]: 0
[agent_80]: 0
[agent_10]: 0
[agent_90]: 0
[agent_30]: 0
[agent_20]: 0
[agent_60]: 0
[agent_00]: 0
[agent_40]: 0
[agent_70]: 0
```

Closes #2948

Co-authored-by: Kevin Qian <kqian@google.com>
PiperOrigin-RevId: 826187198

src/google/adk/flows/llm_flows/contents.py

@@ -594,7 +594,12 @@ def _is_event_belongs_to_branch(
   """
   if not invocation_branch or not event.branch:
     return True
-  return invocation_branch.startswith(event.branch)
+  # We use dot to delimit branch nodes. To avoid simple prefix match
+  # (e.g. agent_0 unexpectedly matching agent_00), require either perfect branch
+  # match, or match prefix with an additional explicit '.'
+  return invocation_branch == event.branch or invocation_branch.startswith(
+      f'{event.branch}.'
+  )
 
 
 def _is_function_call_event(event: Event, function_name: str) -> bool:

tests/unittests/flows/llm_flows/test_contents_branch.py

@@ -58,6 +58,18 @@ async def test_branch_filtering_child_sees_parent():
           content=types.ModelContent("Child agent response"),
           branch="parent_agent.child_agent",  # Current branch - should be included
       ),
+      Event(
+          invocation_id="inv4",
+          author="child_agent",
+          content=types.ModelContent("Excluded response 1"),
+          branch="parent_agent.child_agent000",  # Prefix match BUT not itself/ancestor - should be excluded
+      ),
+      Event(
+          invocation_id="inv5",
+          author="child_agent",
+          content=types.ModelContent("Excluded response 2"),
+          branch="parent_agent.child",  # Prefix match BUT not itself/ancestor - should be excluded
+      ),
   ]
   invocation_context.session.events = events