feat: complete proxy reliability and observability

[KochC]

Summary

• preserve cross-provider conversation semantics through a canonical history layer and standalone bundled plugin
• add bounded tool backpressure, explicit stream disposal, improved Gemini tool streaming, strict model routing, and real-socket lifecycle handling
• add authenticated Prometheus metrics and opt-in SSRF-safe remote media fetching
• add protocol conformance fixtures, HTTP integration coverage, CI checks, and expanded security/configuration documentation

Verification

• npm test (240 passed)
• npm run test:conformance (4 passed)
• npm run test:integration (6 passed)
• npm run lint
• npm run build
• npm audit (0 vulnerabilities)
• npm pack --dry-run
• git diff --check