feat: migrate to Praxis filter-based proxy architecture

[franciscojavierarceo]

Summary

Replaces the hand-rolled Axum proxy with Praxis, a composable filter-based reverse proxy framework built on Pingora. Each gateway concern — proxying, auth, state hydration, tool dispatch, agentic looping — is an independent filter wired together via YAML configuration.

Why Praxis

• Each filter is self-contained — implements HttpFilter with hooks for request/response. Filters don't know about each other.
• YAML-configured pipeline — adding, removing, or reordering filters requires no code changes.
• Native SSE streaming — Praxis/Pingora proxies the upstream response stream directly to the client. No buffering, no reqwest intermediary.
• Hot reload — filter pipelines can be reloaded without restarting the server.

What changed

Filters introduced:

• responses_proxy — sets ctx.upstream to vLLM's /v1/responses endpoint and injects auth credentials. Praxis/Pingora handles the actual proxying and streaming natively.
• state_hydration — stub filter for conversation-state hydration. Inspects request body for previous_response_id and will call the state store to hydrate conversation history.
• agentic_loop — stub filter for agentic re-inference. Inspects response body for function_call output items and will re-enter the inference loop.
• tool_dispatch — stub filter for tool execution. Inspects response body for tool calls and will dispatch them.

Removed:

• src/app.rs, src/proxy.rs, src/server.rs — replaced by filters + Praxis server runtime
• benches/proxy_bench.rs — benchmark harness for the old Axum proxy (will be re-added)

Dependencies:

• Praxis crates (praxis, praxis-proxy-core, praxis-proxy-filter, praxis-test-utils) via git at rev 2f7ea31
• Base URLs ending with /v1 are normalized to avoid /v1/v1/responses double-prefix

Docs:

• Updated README.md with architecture diagram, filter table, and run instructions
• Updated docs/index.md with architecture overview and Praxis context
• Added docs/architecture/index.md with Mermaid diagram, filter pipeline reference, streaming details, and component descriptions
• Added Architecture page to mkdocs nav

Health endpoint:

• Provided by Praxis's built-in admin endpoint (admin: { address: "127.0.0.1:9901" } in config)

Filter pipeline

filter_chains:
  - name: agentic
    filters:
      - filter: state_hydration
        store_base_url: "http://localhost:8080"
      - filter: agentic_loop
        max_iterations: 10
      - filter: tool_dispatch
      - filter: responses_proxy
        vllm_base_url: "http://localhost:8000"

Test plan

• cargo build succeeds
• cargo clippy --all-targets -- -D warnings clean
• cargo fmt -- --check clean
• pre-commit run --all-files clean
• All 9 tests pass (2 unit + 7 integration):
  • test_non_stream_passthrough — JSON request/response round-trip
  • test_stream_passthrough — SSE streaming passthrough
  • test_auth_injection — API key injected from config
  • test_client_auth_precedence — client-supplied auth preserved
  • test_vllm_http_error_passthrough — upstream 429 forwarded
  • test_mid_stream_failure_closes_cleanly — partial stream handled
  • test_connect_error_maps_to_502 — unreachable vLLM returns 502