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/cli/utils/test_agent_loader.py

@@ -286,7 +286,7 @@ class TestAgentLoader:
       loader = AgentLoader(temp_dir)
       agents_dir = temp_dir  # For use in the expected message string
 
-      # Try to load non-existent agent
+      # Try to load nonexistent agent
       with pytest.raises(ValueError) as exc_info:
         loader.load_agent("nonexistent_agent")
 
@@ -328,12 +328,12 @@ class TestAgentLoader:
       assert "No root_agent found for 'broken_agent'" in str(exc_info.value)
 
   def test_agent_internal_module_not_found_error(self):
-    """Test error when an agent tries to import a non-existent module."""
+    """Test error when an agent tries to import a nonexistent module."""
     with tempfile.TemporaryDirectory() as temp_dir:
       temp_path = Path(temp_dir)
       agent_name = "importer_agent"
 
-      # Create agent that imports a non-existent module
+      # Create agent that imports a nonexistent module
       agent_file = temp_path / f"{agent_name}.py"
       agent_file.write_text(dedent(f"""
                 from google.adk.agents.base_agent import BaseAgent
@@ -526,7 +526,7 @@ class TestAgentLoader:
       loader = AgentLoader(temp_dir)
       agents_dir = temp_dir  # For use in the expected message string
 
-      # Try to load non-existent YAML agent
+      # Try to load nonexistent YAML agent
       with pytest.raises(ValueError) as exc_info:
         loader.load_agent("nonexistent_yaml_agent")