refactor: cortex talks to neurons instead of mistral.rs directly

Replace NodeConfig (static vram_mb, pinned) with NeuronEndpoint.
Hardware discovery and model pinning now come from neuron API and
models.toml catalogue respectively.

- config.rs: nodes -> neurons, add models_config path
- catalogue.rs: ModelProfile with pinned_on, ModelCatalogue
- poller.rs: poll neuron GET /models (ModelInfo format)
- router.rs: resolve inference endpoint via neuron GET /models/{id}/endpoint
- evictor.rs: call neuron POST /models/unload
- node.rs: remove vram_mb, pinned fields (come from discovery/catalogue)
- All 22 gateway tests updated to mock neuron API
- Remove MistralModelsResponse, ModelLifecycleRequest (no longer needed)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/cortex-gateway/tests/proxy_basic.rs

@@ -4,7 +4,7 @@ use serde_json::json;
 
 #[tokio::test]
 async fn test_chat_completion_proxy() {
-    let mock_url = common::spawn_mock_backend().await;
+    let mock_url = common::spawn_mock_neuron().await;
     let gw_url = common::spawn_gateway(&mock_url).await;
 
     let client = reqwest::Client::new();
@@ -33,7 +33,7 @@ async fn test_chat_completion_proxy() {
 
 #[tokio::test]
 async fn test_health_endpoint() {
-    let mock_url = common::spawn_mock_backend().await;
+    let mock_url = common::spawn_mock_neuron().await;
     let gw_url = common::spawn_gateway(&mock_url).await;
 
     let client = reqwest::Client::new();
@@ -53,7 +53,7 @@ async fn test_health_endpoint() {
 
 #[tokio::test]
 async fn test_list_models() {
-    let mock_url = common::spawn_mock_backend().await;
+    let mock_url = common::spawn_mock_neuron().await;
     let gw_url = common::spawn_gateway(&mock_url).await;
 
     let client = reqwest::Client::new();
@@ -75,7 +75,7 @@ async fn test_list_models() {
 
 #[tokio::test]
 async fn test_model_not_found() {
-    let mock_url = common::spawn_mock_backend().await;
+    let mock_url = common::spawn_mock_neuron().await;
     let gw_url = common::spawn_gateway(&mock_url).await;
 
     let client = reqwest::Client::new();
@@ -112,12 +112,11 @@ async fn test_no_healthy_nodes() {
             strategy: cortex_core::config::EvictionStrategy::Lru,
             defrag_after_cycles: 0,
         },
-        nodes: vec![cortex_core::config::NodeConfig {
+        neurons: vec![cortex_core::config::NeuronEndpoint {
             name: "dead-node".into(),
             endpoint: "http://127.0.0.1:1".into(),
-            vram_mb: 24000,
-            pinned: vec![],
         }],
+        models_config: "/dev/null".into(),
     };
     let fleet = std::sync::Arc::new(cortex_gateway::state::CortexState::from_config(&config));
 
@@ -153,7 +152,7 @@ async fn test_no_healthy_nodes() {
 
 #[tokio::test]
 async fn test_missing_model_field() {
-    let mock_url = common::spawn_mock_backend().await;
+    let mock_url = common::spawn_mock_neuron().await;
     let gw_url = common::spawn_gateway(&mock_url).await;
 
     let client = reqwest::Client::new();