refactor(async-tool-messages): unify on AsyncToolMessagePayload, fix JSON shape on the wire

Reshape the helper module so AsyncToolMessagePayload is the canonical
in-memory form and the on-the-wire JSON is always derived from it
(never stored). This eliminates a drift risk that came with caching
the JSON in raw_content, and it lets prepare_message_payload_for_realtime
edit the payload (graft the re-invocation reminder into 'description')
and then serialize cleanly — which fixes a 'Tool Response parsing error'
from AWS Nova Sonic that was caused by wrapping the JSON with extra
prose.

Other changes:

- Builders construct an AsyncToolMessagePayload internally and convert
  via shared private _payload_to_message and _payload_to_json helpers
  (centralizing field-omission rules, e.g. no 'result' on 'started').
- prepare_message_payload_for_realtime replaces format_text_for_provider,
  dispatching to per-kind helpers. Reminder is now appended after the
  canonical description so the model reads the protocol explanation
  first and the directive flows from it.
- Final-result payloads are pass-through; the task is done at that
  point and re-invocation is no longer a mistake.
- Stream-tool example: lengthen intermediate sleeps 10s → 20s for more
  interesting empirical testing.

examples/realtime/realtime-aws-nova-sonic-async-stream-tool.py

@@ -54,14 +54,14 @@ async def track_current_location(params: FunctionCallParams):
         properties=FunctionCallResultProperties(is_final=False),
     )
 
-    await asyncio.sleep(10)
+    await asyncio.sleep(20)
     gps = {"lat": 33.96003, "lng": -118.40639}
     await params.result_callback(
         {"gps": gps, "city": "Los Angeles"},
         properties=FunctionCallResultProperties(is_final=False),
     )
 
-    await asyncio.sleep(10)
+    await asyncio.sleep(20)
     gps = {"lat": 32.743569, "lng": -117.20466}
     await params.result_callback({"gps": gps, "city": "San Diego"})