fix(gemini): preserve per-tool-call thought signatures

[macro-ss]

Gemini 3 thinking models attach a thoughtSignature to each function-call
part that must be replayed verbatim on that specific part. crush stored a
single ReasoningContent per assistant message and concatenated every
signature into one string (AppendThoughtSignature), losing per-call
association. On replay ToAIMessage sent the concatenated blob as one
signature, so Gemini rejected the request with 'Corrupted thought
signature'. The corrupted signature was persisted, poisoning the whole
session and recurring across restarts.

• message.ToolCall: add ThoughtSignature field; preserve it in FinishToolCall/AppendToolCallInput; add SetToolCallThoughtSignature
• stop reasoning mutators (FinishThinking, AppendReasoningContent, SetReasoningResponsesData) from silently dropping signature fields
• ToAIMessage: emit each signature in its own ReasoningPart immediately before its text/tool-call part instead of concatenating
• agent: buffer per-toolID signatures in OnReasoningEnd and attach them to the matching tool call in OnToolCall

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com

• I have read CONTRIBUTING.md.
• I have created a discussion that was approved by a maintainer (for new features).

[charmcli]

Thank you for your submission. We really appreciate it! Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request comment same as the below format.

---

I have read the Contributor License Agreement (CLA) and hereby sign the CLA.

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}

[macro-ss]

bugfix for the same issue: #1698

[macro-ss]

I have read the Contributor License Agreement (CLA) and hereby sign the CLA.

[macro-ss]

recheck