feat: make LlmAgent.model optional with a default fallback

LlmAgent now resolves model from ancestors or a system default (gemini-2.5-flash) when unset. Added LlmAgent.set_default_model() to override the default globally

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 853006116

tests/unittests/flows/llm_flows/test_basic_processor.py

@@ -110,7 +110,9 @@ class TestBasicLlmRequestProcessor:
     assert llm_request.config.response_mime_type != 'application/json'
 
     # Should have checked if output schema can be used with tools
-    can_use_output_schema_with_tools.assert_called_once_with(agent.model)
+    can_use_output_schema_with_tools.assert_called_once_with(
+        agent.canonical_model
+    )
 
   @pytest.mark.asyncio
   async def test_sets_output_schema_when_tools_present(self, mocker):
@@ -141,7 +143,9 @@ class TestBasicLlmRequestProcessor:
     assert llm_request.config.response_mime_type == 'application/json'
 
     # Should have checked if output schema can be used with tools
-    can_use_output_schema_with_tools.assert_called_once_with(agent.model)
+    can_use_output_schema_with_tools.assert_called_once_with(
+        agent.canonical_model
+    )
 
   @pytest.mark.asyncio
   async def test_no_output_schema_no_tools(self):

tests/unittests/flows/llm_flows/test_output_schema_processor.py

@@ -191,7 +191,9 @@ async def test_output_schema_request_processor(
     assert not llm_request.config.system_instruction
 
   # Should have checked if output schema can be used with tools
-  can_use_output_schema_with_tools.assert_called_once_with(agent.model)
+  can_use_output_schema_with_tools.assert_called_once_with(
+      agent.canonical_model
+  )
 
 
 @pytest.mark.asyncio