fix(ai): avoid unsigned toolCall parts for Gemini 3 history

[roshanasingh4]

Fixes a Gemini 3 validation error when replaying history containing tool calls without thoughtSignature (e.g. when the previous turn came from Claude via Antigravity).

Approach:

• In packages/ai/src/providers/google-shared.ts detect gemini-3* models and, when a tool call has no thoughtSignature, serialize the tool call as plain text instead of emitting a functionCall part.
• Keep signed tool calls unchanged (still emitted as functionCall).

Regression test:

• Added packages/ai/test/google-shared-gemini3-unsigned-tool-call.test.ts to assert unsigned tool calls become text parts for Gemini 3.

Context: moltbot/moltbot#922

[badlogic]

Cheers!