fix: dont overwrite thought signatures

[markmcd]

Description

In chat_from_gemini_response, thought_sigatures can be overwritten with None if the list of parts & signatures gets manually edited.

This is unlikely to happen under normal usage but if, say, a user has edited the block list and not kept the additional thought_signatures list in sync, or removed it, then any existing ThinkingBlocks would get their thought signatures set to None.

Version Bump?

• Yes

Type of Change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• I added new unit tests to cover this change
• I believe this change is already covered by existing unit tests

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I ran uv run make format; uv run make lint to appease the lint gods