fix(streaming): guard stop_* fields from being overwritten by subsequent message_delta events

[iosayin]

Fixes #1940

Summary

In streaming message accumulation (_messages.py and _beta_messages.py), subsequent message_delta events that omit stop_reason, stop_sequence, or stop_details (i.e., their delta values are None) previously overwrote the fields in current_snapshot unconditionally. This caused accumulated stop metadata from earlier deltas or start events to be reset to None.

Changes

• Added is not None guards for event.delta.stop_reason, event.delta.stop_sequence, and event.delta.stop_details in both src/anthropic/lib/streaming/_messages.py and src/anthropic/lib/streaming/_beta_messages.py, matching the convention of container, input_transformations, and optional usage fields.
• Added regression tests in tests/lib/streaming/test_messages.py and tests/lib/streaming/test_beta_messages.py verifying that omitting stop metadata on subsequent deltas preserves earlier values.
• Formatted and linted with ruff.
• Cryptographically signed commit with SSH key.

[sigley]

Validated exact head c4808ae3 independently. A later usage-only message_delta currently clears an already accumulated stop_reason / stop_sequence; this head preserves those fields when the incoming delta leaves them unset, while still applying new non-null values. A two-delta synthetic sequence reproduces the loss on current main and preserves the terminal metadata on this head. The branch merges cleanly with current main, and the stable + beta streaming test slices pass 69/69. I do not see a blocking correctness issue.