fix: use get_final_message() to prevent AttributeError on early stream termination

Fixes #1868

When the Anthropic API stream terminates before sending message_stop
(e.g. network timeout, connection reset), the last event in the
async for loop may not have a .message attribute, causing:

  AttributeError: 'TextEvent' object has no attribute 'message'

Replace the fragile event.message.usage access with
stream.get_final_message() which safely returns the accumulated
message snapshot regardless of how the stream ended. Falls back to
the old pattern for test/mock streams that lack this API.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: giulio-leone

src/strands/models/anthropic.py

@@ -409,7 +409,15 @@ async def stream(
                     if event.type in AnthropicModel.EVENT_TYPES:
                         yield self.format_chunk(event.model_dump())
 
-                usage = event.message.usage  # type: ignore
+                # Prefer get_final_message() which safely handles early stream
+                # termination (e.g. network timeout before message_stop).
+                # Fall back to the last event for mock/test streams that lack
+                # the get_final_message API.
+                try:
+                    final_message = await stream.get_final_message()
+                    usage = final_message.usage
+                except (AttributeError, Exception):
+                    usage = event.message.usage  # type: ignore
                 yield self.format_chunk({"type": "metadata", "usage": usage.model_dump()})
 
         except anthropic.RateLimitError as error: