chore: Fix spelling

Merge https://github.com/google/adk-python/pull/2447

This PR corrects misspellings identified by the [check-spelling action](https://github.com/marketplace/actions/check-spelling)

The misspellings have been reported at https://github.com/jsoref/adk-python/actions/runs/16840838898/attempts/1#summary-47711379253

The action reports that the changes in this PR would make it happy: https://github.com/jsoref/adk-python/actions/runs/16840839269/attempts/1#summary-47711380479

Note: while I use tooling to identify errors, the tooling doesn't _actually_ provide the corrections, I'm picking them on my own. I'm a human, and I may make mistakes.

I've included a couple of changes to make CI happy. Personally, I object to CI being in a state of "random drive by person who adds a blank line in the middle of a file must fix all the preexisting bugs in the file", but that appears to be the state for this repository.

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/2447 from jsoref:spelling d85398e7fd154d124d477c6af6181481a01f34e0
PiperOrigin-RevId: 827629615

tests/unittests/agents/test_agent_clone.py

@@ -274,7 +274,7 @@ def test_clone_invalid_field():
   """Test that cloning with invalid fields raises an error."""
   original = LlmAgent(name="test_agent", description="Test agent")
 
-  with pytest.raises(ValueError, match="Cannot update non-existent fields"):
+  with pytest.raises(ValueError, match="Cannot update nonexistent fields"):
     original.clone(update={"invalid_field": "value"})
 
 

tests/unittests/agents/test_remote_a2a_agent.py

@@ -360,7 +360,7 @@ class TestRemoteA2aAgentResolution:
 
   @pytest.mark.asyncio
   async def test_resolve_agent_card_from_file_not_found(self):
-    """Test agent card resolution from non-existent file raises error."""
+    """Test agent card resolution from nonexistent file raises error."""
     agent = RemoteA2aAgent(
         name="test_agent", agent_card="/path/to/nonexistent.json"
     )