fix(gemini): set both camelCase and snake_case thought_signature in tool call request body

[ZOOWH]

Summary

• The Gemini HTTP API provider only set thoughtSignature (camelCase) when building tool call request bodies
• Gemini 2.5 models accept camelCase, but Gemini 3.5 Flash Agentic reasoning requires thought_signature (snake_case) — matching the format it returns in API responses
• When only the camelCase variant was sent, Gemini 3.5 Flash rejected the request with a 400 Bad Request for missing thought_signature
• The fix sets both ThoughtSignature and ThoughtSignatureSnake in the geminiPart struct, matching the pattern already used in antigravity_provider.go

Verification

• Ran go test ./pkg/providers/httpapi/... -count=1 — all tests pass
• Updated test from TestGeminiProvider_BuildRequestBody_UsesCamelCaseThoughtSignatureOnly to TestGeminiProvider_BuildRequestBody_SetsBothThoughtSignatureFormats to validate both formats are present

Real behavior proof

Behavior addressed: Gemini 3.5 Flash Agentic tool calls no longer fail with 400 Bad Request for missing thought_signature; both camelCase and snake_case variants are now included in the request body

Environment tested: Go 1.25.10 on Linux amd64, go test ./pkg/providers/httpapi/...

Steps run after the patch: Ran targeted unit tests for the httpapi provider

Evidence after fix:

ok  github.com/sipeed/picoclaw/pkg/providers/httpapi  0.029s

Observed result after the fix: All httpapi provider tests pass. Request body now contains both "thoughtSignature":"sig-1" and "thought_signature":"sig-1", ensuring compatibility with both Gemini 2.x and Gemini 3.x Agentic models.

Not tested: Live Gemini 3.5 Flash Agentic API request with real tool calls

Root Cause

• Root cause: gemini_provider.go only set part.ThoughtSignature (camelCase json tag) in the request body, without also setting part.ThoughtSignatureSnake (snake_case json tag)
• Missing detection/guardrail: antigravity_provider.go already sets both formats (lines 267-268), but the Gemini direct HTTP API provider was missing the snake_case variant

Closes #3111

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