Fix `ThinkingPartDelta.apply(ThinkingPart)` not updating `ThinkingPart.signature`

[CallumJHays]

Initial Checks

• I confirm that I'm using the latest version of Pydantic AI
• I confirm that I searched for my issue in https://github.com/pydantic/pydantic-ai/issues before opening this issue

Description

Hi 👋 I came across an issue while trying to subclass BedrockConverseModel to enable mapping ThinkingPart to ReasoningContentBlockOutputTypeDef:

ThinkingPartDelta.signature_delta does not update ThinkingPart during ThinkingPartDelta.apply, and gets discarded. This issue might apply to other models, not just bedrock.

pydantic-ai/pydantic_ai_slim/pydantic_ai/_parts_manager.py

Lines 191 to 193 in 9c7480d

	part_delta = ThinkingPartDelta(signature_delta=signature)
	self._parts[part_index] = part_delta.apply(existing_thinking_part)
	return PartDeltaEvent(index=part_index, delta=part_delta)

pydantic-ai/pydantic_ai_slim/pydantic_ai/messages.py

Lines 765 to 766 in 9c7480d

	if isinstance(part, ThinkingPart):
	return replace(part, content=part.content + self.content_delta if self.content_delta else None)

Python, Pydantic AI & LLM client version

Python: 3.12
Pydantic-AI: 0.3.0
LLM: `boto3==1.38.33`

[Kludex]

Thanks @CallumJHays ! My bad... 👀

• Would you be kind to try fix: update ThinkingPart when delta contains signature #2012 ?

[Kludex]

I've preemptively merged it. I'll make a release.

[CallumJHays]

Thanks @Kludex !