fix(ai): patch Gemini tool thought signatures

[codedogQBY]

Analysis

• [Bug] Gemini Ai Error 总结当前章节 #225 reports Gemini 3 returning 400 Function call is missing a thought_signature after the reader AI calls getCurrentChapter.
• Google documents that Gemini 3 function-calling turns must send a tool_calls[].extra_content.google.thought_signature value back on the first function call part in each step.
• LangChain's OpenAI-compatible conversion reserializes parsed tool calls and can drop provider-specific extra_content, so the next Gemini/OpenAI-compatible request can fail before the tool result is processed.

Changes

• Patch Gemini/OpenAI-compatible POST request bodies in the endpoint fetch wrapper before sending them.
• Preserve existing Gemini thought signatures when present.
• Add the documented skip_thought_signature_validator fallback to missing first function tool calls for Gemini 3 / Google-compatible requests.
• Add focused tests for Gemini 3 custom endpoints, existing signatures, and non-Gemini requests.

Verification

• pnpm --filter @readany/core test -- llm-provider
• pnpm --filter @readany/core exec tsc --noEmit
• pnpm exec biome check packages/core/src/ai/llm-provider.ts packages/core/src/ai/__tests__/llm-provider.test.ts (passes with existing DeepSeek override warnings in llm-provider.ts)
• git diff --check

Fixes #225