Add native Gemini provider support

[adilhafeez]

Summary

• Add native Gemini GenerateContent API types, request/response transforms, and provider routing to hermesllm
• Support bidirectional translation between Gemini and all existing client/upstream APIs (OpenAI, Anthropic, Bedrock, Responses)
• Include streaming support via :streamGenerateContent?alt=sse endpoints
• Add 164 passing unit tests covering serde, transforms, and routing

Test plan

• cargo test --lib — all 164 hermesllm tests pass, full crate suite passes
• cargo clippy --locked --all-targets --all-features -- -D warnings — clean
• cargo fmt --all -- --check — clean
• cargo build --release --target=wasm32-wasip1 -p llm_gateway -p prompt_gateway — WASM plugins build
• cargo build --release -p brightstaff — native binary builds
• CI checks pass on GitHub

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