fix: preserve Gemini thought_signature across multi-turn tool calls

[Copilot]

Gemini 2.5 thinking models return a thoughtSignature on thought parts that must be echoed back verbatim in every subsequent conversation turn. The agent loop was silently dropping it, causing HTTP 400 INVALID_ARGUMENT on any multi-turn tool use with gemini-2.5-flash / gemini-2.5-pro.

Changes

• convertGoogleResponse() — preserve part.thoughtSignature → _googleThoughtSignature on the internal thinking block; also handle part.thought parts that have no text (signature-only parts are valid)
• streamGoogleResponse() / yieldChunkEvents() — branch on part.thought (not part.text) first; forward thoughtSignature on emitted thinking_delta events
• accumulateGoogleStream() — collect signature from any streaming chunk and store it on the thinking block; create a thinking block even when thinkingContent is empty but a signature is present
• callGoogle() request builder — re-emit { thought: true, thoughtSignature: ... } parts before functionCall parts when a thinking block carries _googleThoughtSignature

The internal field name uses the underscore-prefix convention to distinguish it from the wire-format field thoughtSignature.

// Before: model turn reconstructed without the required thought context
{ role: 'model', parts: [{ functionCall: { name: 'LS', args: {} } }] }
//  → 400: "Function call is missing a thought_signature"

// After: thought part re-emitted alongside the function call
{ role: 'model', parts: [
  { thought: true, thoughtSignature: 'CiQKIg...' },
  { functionCall: { name: 'LS', args: {} } },
] }