Prefer init-provided system instruction in Grok Realtime

Add system_instruction parameter to the Grok Realtime adapter's
get_llm_invocation_params() and call _resolve_system_instruction() to
prefer init-provided over context-provided system instructions and
warn on conflicts. Previously context-provided took precedence.

Update the Grok Realtime example to use settings.system_instruction
instead of session_properties.instructions.

examples/foundational/51-grok-realtime.py

@@ -172,23 +172,6 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
     session_properties = SessionProperties(
         # Voice options: Ara, Rex, Sal, Eve, Leo
         voice="Ara",
-        # System instructions
-        instructions="""You are a helpful and friendly AI assistant powered by Grok.
-
-You have access to several tools:
-- Weather information
-- Current time
-- Restaurant recommendations
-- Web search (built-in)
-- X/Twitter search (built-in)
-
-Your voice and personality should be warm and engaging. Keep your responses
-concise and conversational since this is a voice interaction.
-
-If the user asks about current events or news, use web search.
-If they ask about what people are saying on social media, use X search.
-
-Always be helpful and proactive in offering assistance.""",
         # Grok-specific built-in tools can be added here:
         # tools=[
         #     WebSearchTool(),  # Enable web search
@@ -200,6 +183,22 @@ Always be helpful and proactive in offering assistance.""",
     llm = GrokRealtimeLLMService(
         api_key=os.getenv("GROK_API_KEY"),
         settings=GrokRealtimeLLMService.Settings(
+            system_instruction="""You are a helpful and friendly AI assistant powered by Grok.
+
+    You have access to several tools:
+    - Weather information
+    - Current time
+    - Restaurant recommendations
+    - Web search (built-in)
+    - X/Twitter search (built-in)
+
+    Your voice and personality should be warm and engaging. Keep your responses
+    concise and conversational since this is a voice interaction.
+
+    If the user asks about current events or news, use web search.
+    If they ask about what people are saying on social media, use X search.
+
+    Always be helpful and proactive in offering assistance.""",
             session_properties=session_properties,
         ),
     )