fix(anthropic): use get_final_message for robust usage extraction after stream

[giulio-leone]

Summary

Fixes #1868
Related: #1746, #1785

When the Anthropic stream terminates before emitting a message_stop event (e.g. on server disconnect or early stop), the last loop variable event may be a TextEvent that has no .message attribute, causing:

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

Root Cause

After the async for event in stream loop, the code accessed event.message.usage which relies on event being the last yielded event. If the stream terminates early, event may not have a .message attribute.

Fix

Replace event.message.usage with await stream.get_final_message() which safely returns the accumulated message regardless of how the stream ended. This is the recommended Anthropic SDK pattern for extracting final metadata.

Testing

• Updated test_stream to use a mock stream with get_final_message()
• Added test_stream_early_termination covering the case where no recognized events are yielded before the stream closes

Both tests pass locally.

[giulio-leone]

Friendly ping — this fixes Anthropic usage extraction after streaming by using get_final_message() instead of indexing into content blocks, which can miss usage data on partial responses.

[giulio-leone]

PR refresh — 2026-03-23

Cherry-picked onto current main (fd8168a5) from prior merge-base fca208b8. Branch fix/anthropic-stream-early-termination force-pushed to fork.

Test validation (double-pass):

• tests/strands/models/test_anthropic.py
• Pass 1: 42/42 ✅
• Pass 2: 42/42 ✅

All tests pass. PR is ready for review.