feat: harden and extend the LLM proxy

[KochC]

Summary

• add request lifecycle controls, session cleanup, bounded concurrency, safer tool-bridge handling, and streaming cancellation
• harden authentication, CORS, LAN startup, media handling, errors, response headers, MCP framing, dependencies, and CI
• add structured output, embedded multimodal inputs, generation controls, model variants, aliases/fallbacks, and richer model metadata
• expand protocol and regression coverage to 184 tests

Verification

• npm test (184 passed)
• npm run lint
• npm audit (0 vulnerabilities)
• npm pack --dry-run
• git diff --check