feat(authbridge): Praxis integration — ext_proc drop-in, then native-Rust filters

[huang195]

From the AuthBridge roadmap (Deep Dive → Next Steps · Exploration).

Explore moving the data plane to Praxis (Rust) in two stages:

• Light — Praxis owns the data plane + protocol parsing; AuthBridge consumes the parsed result over ext_proc gRPC (no rewrite). (ext_proc in Praxis is still draft.)
• Deep — AuthBridge plugins as native Rust filters in-process on Praxis; drop the Go A2A/MCP/inference parsers.

Goal: smaller/faster proxy, semantic parsing built in. Tracking issue for design + spike.

[esnible]

Closing in favor of #681