gateway/v1: model-prefix routing closes Phase 39 PRD gate

Phase 39 PRD (docs/CONTROL_PLANE_PRD.md:62) promised:
  "/v1/chat routes by `model` field: prefix match
   (e.g. openrouter/anthropic/claude-3.5-sonnet → OpenRouter;
   bare names → Ollama)"

Actual behavior required clients to pass `provider: "openrouter"`
explicitly. Bare `model: "openrouter/..."` would fall through to the
"unknown provider ''" error. PRD gate never actually passed.

Fix: resolve_provider(&ChatRequest) picks (provider, effective_model):
  - explicit `req.provider` wins, model passes through unchanged
  - else strip "openrouter/" prefix → provider="openrouter", model
    without prefix (OpenRouter API expects "openai/gpt-4o-mini",
    not "openrouter/openai/gpt-4o-mini")
  - else strip "cloud/" prefix → provider="ollama_cloud"
  - else default provider="ollama"

Adapter calls use Cow<ChatRequest>: borrowed when no strip needed
(zero alloc), owned when we needed to build a new model string. Keeps
the hot path allocation-free for the common case.

ChatRequest gains #[derive(Clone)] — needed for the Owned variant.
5 new tests pin the resolution semantics including the
"explicit provider + prefixed model" corner case (trust the caller,
don't double-strip).

Workspace warnings unchanged at 0.

Still not shipped from Phase 39: config/providers.toml — hardcoded
match arms work fine in practice, centralizing them is cosmetic.
Flag as a follow-up if a 4th provider lands.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/gateway/src/v1/mod.rs

@@ -84,7 +84,7 @@ pub struct Message {
     pub content: String,
 }
 
-#[derive(Deserialize, Debug)]
+#[derive(Deserialize, Debug, Clone)]
 pub struct ChatRequest {
     pub model: String,
     pub messages: Vec<Message>,
@@ -140,6 +140,78 @@ pub struct UsageBlock {
 
 // -- Handlers --
 
+/// Phase 39: resolve (provider, effective_model) from a ChatRequest.
+///
+/// Explicit `req.provider` wins. If absent, infer from a model-name
+/// prefix: "openrouter/..." → openrouter (strip prefix), "cloud/..." →
+/// ollama_cloud (strip prefix). Bare names default to "ollama".
+///
+/// The stripped model is what the upstream adapter expects:
+/// OpenRouter's API wants "openai/gpt-4o-mini", not
+/// "openrouter/openai/gpt-4o-mini".
+fn resolve_provider(req: &ChatRequest) -> (String, String) {
+    if let Some(p) = req.provider.as_deref() {
+        return (p.to_ascii_lowercase(), req.model.clone());
+    }
+    if let Some(rest) = req.model.strip_prefix("openrouter/") {
+        return ("openrouter".to_string(), rest.to_string());
+    }
+    if let Some(rest) = req.model.strip_prefix("cloud/") {
+        return ("ollama_cloud".to_string(), rest.to_string());
+    }
+    ("ollama".to_string(), req.model.clone())
+}
+
+#[cfg(test)]
+mod resolve_provider_tests {
+    use super::*;
+
+    fn mk_req(provider: Option<&str>, model: &str) -> ChatRequest {
+        ChatRequest {
+            model: model.to_string(),
+            messages: vec![],
+            temperature: None,
+            max_tokens: None,
+            stream: None,
+            think: None,
+            provider: provider.map(|s| s.to_string()),
+        }
+    }
+
+    #[test]
+    fn explicit_provider_wins() {
+        let r = mk_req(Some("openrouter"), "qwen3.5:latest");
+        assert_eq!(resolve_provider(&r), ("openrouter".into(), "qwen3.5:latest".into()));
+    }
+
+    #[test]
+    fn bare_model_defaults_to_ollama() {
+        let r = mk_req(None, "qwen3.5:latest");
+        assert_eq!(resolve_provider(&r), ("ollama".into(), "qwen3.5:latest".into()));
+    }
+
+    #[test]
+    fn openrouter_prefix_infers_and_strips() {
+        let r = mk_req(None, "openrouter/openai/gpt-4o-mini");
+        assert_eq!(resolve_provider(&r), ("openrouter".into(), "openai/gpt-4o-mini".into()));
+    }
+
+    #[test]
+    fn cloud_prefix_infers_and_strips() {
+        let r = mk_req(None, "cloud/kimi-k2:1t");
+        assert_eq!(resolve_provider(&r), ("ollama_cloud".into(), "kimi-k2:1t".into()));
+    }
+
+    #[test]
+    fn explicit_provider_preserves_full_model_even_with_prefix() {
+        // If caller provides both provider and a model with a prefix,
+        // trust them — don't strip. The adapter will get the full model
+        // string as-is.
+        let r = mk_req(Some("openrouter"), "openrouter/openai/gpt-4o-mini");
+        assert_eq!(resolve_provider(&r), ("openrouter".into(), "openrouter/openai/gpt-4o-mini".into()));
+    }
+}
+
 async fn chat(
     State(state): State<V1State>,
     Json(req): Json<ChatRequest>,
@@ -151,13 +223,29 @@ async fn chat(
         tracing::warn!("/v1/chat: stream=true requested but Phase 38 returns non-streaming");
     }
 
-    let provider = req.provider.as_deref().unwrap_or("ollama").to_ascii_lowercase();
+    // Provider resolution: explicit `req.provider` wins; otherwise
+    // infer from a model-name prefix. Phase 39 PRD gate example:
+    // `model: "openrouter/openai/gpt-4o-mini"` → provider "openrouter",
+    // adapter gets the stripped "openai/gpt-4o-mini".
+    let (provider, effective_model) = resolve_provider(&req);
     let start_time = chrono::Utc::now();
     let start_instant = std::time::Instant::now();
 
+    // If we stripped a prefix, clone req with the effective model so
+    // the adapter sees what the upstream provider expects (OpenRouter
+    // wants "openai/gpt-4o-mini", not "openrouter/openai/gpt-4o-mini").
+    let req_for_adapter: std::borrow::Cow<'_, ChatRequest> =
+        if effective_model == req.model {
+            std::borrow::Cow::Borrowed(&req)
+        } else {
+            let mut cloned = req.clone();
+            cloned.model = effective_model.clone();
+            std::borrow::Cow::Owned(cloned)
+        };
+
     let (resp, used_provider) = match provider.as_str() {
         "ollama" | "local" | "" => {
-            let r = ollama::chat(&state.ai_client, &req)
+            let r = ollama::chat(&state.ai_client, &*req_for_adapter)
                 .await
                 .map_err(|e| (StatusCode::BAD_GATEWAY, format!("ollama local: {e}")))?;
             (r, "ollama".to_string())
@@ -167,7 +255,7 @@ async fn chat(
                 StatusCode::SERVICE_UNAVAILABLE,
                 "OLLAMA_CLOUD_KEY not configured".to_string(),
             ))?;
-            let r = ollama_cloud::chat(key, &req)
+            let r = ollama_cloud::chat(key, &*req_for_adapter)
                 .await
                 .map_err(|e| (StatusCode::BAD_GATEWAY, format!("ollama cloud: {e}")))?;
             (r, "ollama_cloud".to_string())
@@ -180,7 +268,7 @@ async fn chat(
                 StatusCode::SERVICE_UNAVAILABLE,
                 "OPENROUTER_API_KEY not configured".to_string(),
             ))?;
-            let r = openrouter::chat(key, &req)
+            let r = openrouter::chat(key, &*req_for_adapter)
                 .await
                 .map_err(|e| (StatusCode::BAD_GATEWAY, format!("openrouter: {e}")))?;
             (r, "openrouter".to_string())