fix(gemini): migrate to google.golang.org/genai SDK

[nirmal-bluefunda]

Problem

The Gemini provider used github.com/google/generative-ai-go, which Google has deprecated. It can't parse current Gemini 3.x responses:

• Every stream ends in invalid character ']' looking for beginning of value, even after a complete answer, so callers treat good answers as errors.
• Function-calling turns come back as an unknown finish reason (FinishReason(10)) with no content.

Older models that worked with that SDK (e.g. gemini-2.5-flash) are retired for new users, so a model change can't fix it. In cai-llm-router this broke the Gemini model and the tool-retry-fallback (Groq → Gemini) recovery.

Changes

• Provider rewritten on google.golang.org/genai (v1.73.0) in providers/gemini; the old SDK is removed from go.mod.
• Gemini 3 thought signatures: new optional ToolCall.ThoughtSignature field (json:"thought_signature,omitempty"), set from the function-call part and replayed in history. Gemini 3 rejects follow-up tool-loop requests without it. Empty for every other provider.
• Unique tool-call IDs and Index: Gemini often omits call IDs, and the old provider used the function name as the ID, so parallel calls to the same tool collided (cai-llm-router#294).
• Tool history without declared tools is sent as text: Gemini rejects function-call history when the request has no tools (e.g. a tool-free retry).
• Usage reported, with thinking tokens counted as completion tokens (previously missing).
• Tool schemas passed as raw JSON Schema (parametersJsonSchema), so keywords aren't dropped.
• The API's real error and HTTP status are surfaced through APIError.
• Thought parts are skipped (never mixed into answer text). BaseURL and CustomHeaders are honoured.

No other provider changes. Only cai-llm-router depends on this module.

Tests

New providers/gemini/provider_test.go against a fake Gemini server (no key needed):

• a stream ends cleanly, with usage;
• tool calls keep the signature and get unique IDs and indexes;
• thought text never leaks into the answer;
• the follow-up request on the wire replays the signature, groups parallel results, resolves a missing tool name, and keeps the schema intact;
• without tools, tool history is replayed as text;
• API errors (404 with the server message) are reported;
• Complete() works;
• finish-reason mapping.

go vet ./... clean; all packages pass.

Live check: cai-llm-router run locally against this branch (go.mod replace) with the real API and gemini-3.5-flash-lite:

• a plain chat answered cleanly;
• a multi-step tool loop (4 model steps, 3 tool calls) completed with the signatures accepted.

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