fix: Fixes the identity prompt to be one line and add ending period after description statement

From

```
You are an agent. Your internal name is "agent".

 The description about you is "test description"
```

to

```
You are an agent. Your internal name is "agent". The description about you is "test description".
```

PiperOrigin-RevId: 822358196

Author: Jacksunwei

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

@@ -34,10 +34,10 @@ async def run_async(
       self, invocation_context: InvocationContext, llm_request: LlmRequest
   ) -> AsyncGenerator[Event, None]:
     agent = invocation_context.agent
-    si = [f'You are an agent. Your internal name is "{agent.name}".']
+    si = f'You are an agent. Your internal name is "{agent.name}".'
     if agent.description:
-      si.append(f' The description about you is "{agent.description}"')
-    llm_request.append_instructions(si)
+      si += f' The description about you is "{agent.description}".'
+    llm_request.append_instructions([si])
 
     # Maintain async generator behavior
     if False:  # Ensures it behaves as a generator

tests/unittests/flows/llm_flows/test_identity.py

@@ -64,7 +64,8 @@ async def test_with_description():
   ):
     pass
 
-  assert request.config.system_instruction == "\n\n".join([
-      'You are an agent. Your internal name is "agent".',
-      ' The description about you is "test description"',
-  ])
+  assert (
+      request.config.system_instruction
+      == """\
+You are an agent. Your internal name is "agent". The description about you is "test description"."""
+  )