feat: add dry-run route trace/explain endpoint

[gr3enarr0w]

Context

Pi is being moved to use Ferro Gateway as the only model routing plane while retiring SynapseRouter as a runtime router. Pi can already call Ferro through the OpenAI-compatible API, but a Pi-side router UX needs a safe way to ask Ferro why a request would route to a model/provider without actually spending tokens.

Ferro already has routing strategies, request logging, cost metadata, OTel spans, and health checks. This issue is specifically for a client-facing route explanation surface.

Proposal

Add an authenticated dry-run endpoint, for example:

POST /v1/route/trace

Input should accept a chat/completions-shaped request or a reduced route probe:

{
  "model": "qwen3.5:397b-cloud",
  "messages": [{"role": "user", "content": "..."}],
  "stream": false,
  "metadata": {
    "client": "pi-ferro-router"
  }
}

Response should explain the selected target without calling the provider:

{
  "strategy": "conditional",
  "requested_model": "qwen3.5:397b-cloud",
  "selected_target_key": "ollama-cloud",
  "selected_model": "qwen3.5:397b-cloud",
  "candidate_targets": [
    {
      "target_key": "ollama-cloud",
      "matched": true,
      "reason": "model condition matched",
      "healthy": true,
      "circuit_open": false,
      "estimated_cost_usd": null
    }
  ],
  "catalog": {
    "model_found": true,
    "priced": false,
    "context_window": 128000
  }
}

Acceptance Criteria

• Endpoint performs no upstream model call and consumes no provider quota.
• Returns selected target/model and ordered candidate explanations.
• Includes provider health/circuit/retry-relevant state when available.
• Includes model-catalog pricing/capability match state when available.
• Emits normal auth, audit, and OTel/request IDs like other admin/client-visible routes.
• Works for conditional, fallback, cost-optimized, least-latency, and single routing modes.

Non-goals

• Do not add Pi-specific logic to Ferro.
• Do not reintroduce SynapseRouter as a routing dependency.