refactor(async-tool-messages): replace reminder grafting with caller-supplied template

Empirical testing showed the previous design — grafting a verbose
re-invocation reminder into the payload's `description` field for
started and intermediate messages — was actually making Nova Sonic
*worse*, not better: more spurious re-invocations of the same tool,
not fewer. Plausibly the long, instruction-shaped description text
reads as content the model has to respond to, where a terse status
update reads as ambient state.

Replace the reminder grafting with a caller-supplied `template`
keyword argument on `prepare_message_payload_for_realtime`. When
`None` (the default), the payload is serialized to its canonical
JSON form. When provided, `template.format(tool_call_id=…, status=…,
result=…, description=…)` is applied. The template is honored across
all kinds, so callers route per kind based on which wire channel
they're using.

Nova Sonic now defines its own bracketed plain-text template
(`_ASYNC_TOOL_RESULT_TEXT_TEMPLATE`) and applies it on the
cross-modal user-text channel (intermediate / final). The started
path stays on raw JSON (the formal AWS tool-result channel requires
valid JSON). A code comment at the template constant captures the
empirical finding for the next person — short framing yields much
better behavior, surprising as it sounds.

Tests updated for the new template behavior across all kinds. Also
reverts a stream-tool example sleep-duration tweak (20s → 10s) and
adds a commented-out alternative in the function-calling-openai-async-stream
example for parallel testing.

examples/function-calling/function-calling-openai-async-stream.py

@@ -74,6 +74,7 @@ async def track_current_location(params: FunctionCallParams):
 
     # Second update: revised city estimate.
     await asyncio.sleep(10)
+    # await asyncio.sleep(20)
     gps = {"lat": 33.96003, "lng": -118.40639}
     await params.result_callback(
         {"gps": gps, "city": "Los Angeles"},
@@ -82,6 +83,7 @@ async def track_current_location(params: FunctionCallParams):
 
     # Final result: confirmed city.
     await asyncio.sleep(10)
+    # await asyncio.sleep(20)
     gps = {"lat": 32.743569, "lng": -117.20466}
     await params.result_callback({"gps": gps, "city": "San Diego"})