Fix Gemini grounding tool combinations

[Magic-Xin]

Summary

• Enable server-side tool invocation context for Gemini 3.8 Flash so Google Search, Google Maps, and URL Context work together.
• Use Search and URL Context together on Gemini 3.1 Pro; its API rejects Google Maps combined with either tool.
• Remove the deprecated Civic Integrity safety category.
• Preserve the existing message parsing cleanup in gemini.go.

Verification

• Reproduced the original HTTP 400 with the current tool configuration using a fixed minimal prompt and the maintainer-provided API key; the compatible Flash and Pro configurations returned HTTP 200.
• go test ./... passed locally.
• Commit 12b81da has a verified GPG signature.

Merge after the new dev Actions build succeeds. Then move the signed v1.0.0 tag to the resulting main commit and verify the release workflow.