fix(parser): handle missing signature field in ThinkingBlock

[nanookclaw]

Summary

Fixes a KeyError: 'signature' crash in message_parser.py when the API returns a thinking content block without a signature field.

Root Cause

message_parser.py:110 uses a direct dict lookup:

signature=block["signature"],  # KeyError if field absent

The Anthropic API can return thinking blocks without a signature in certain conditions:

• Redacted thinking blocks (policy-based)
• Streaming edge cases
• Older cached responses

Fix

One-line change to use .get() with an empty string default:

signature=block.get("signature", ""),

Test

Added test_parse_assistant_message_with_thinking_missing_signature in the existing test suite — verifies that parsing succeeds (no KeyError) when signature is absent from a thinking block.

All 453 existing tests pass.

Related

Fixes the KeyError: 'signature' reported in #777 by @Zerolzj.