feat(adapters): add Gemini prompt reconstruction

[kev1n77]

Summary

• add a versioned Gemini adapter for v1/v1beta generateContent and streamGenerateContent
• reconstruct system instructions, multimodal contents, function declarations/calls/results, generation options, JSON/SSE responses, finish reasons, and usage metadata
• add capability-matrix fixtures/goldens, desktop provider labeling, and progress tracking

Verification

• cargo fmt --all -- --check
• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --manifest-path apps/desktop/src-tauri/Cargo.toml --all-targets
• cargo clippy --manifest-path apps/desktop/src-tauri/Cargo.toml --all-targets -- -D warnings
• npm run desktop:check
• npm run desktop:build