feat(omp-gemini): optimize Gemini OAuth extension architecture, HTTP/2 pooling & benchmarks

[houenyang-momo]

Summary

• Implement ALPN HTTP/2 persistent connection pooling and eager origin warm-up.
• Add in-memory session cache fast-path in TokenStore.hasSession() to eliminate synchronous macOS Keychain IPC stalls.
• Replace per-chunk Promise.race and setTimeout allocations in SSE stream reader with single sliding watchdog timer.
• Implement model-aware thinking budget mapping (thinkingBudget: 0 for Flash, omitted for Pro to prevent HTTP 400).
• Add fast-path Unicode surrogate sanitizer (isWellFormed) with zero allocations on clean UTF-8.
• Canonical source, regression test suite (23/23 passing), and benchmarks documented under docs/omp-agent/gemini-extension/ and extensions/omp-agent/gemini/.