Acquire and cache opt-in executor capability inventories via V2 (#53736)

## What changed

- Add the disabled-by-default `executor_capability_discovery_v2` feature to acquire `capabilities/discoverV2` inventories from supporting remote executors using each selected environment's working directory and sandbox.
- Share snapshots with MCP and world-state contributors and skill providers without activating discovered plugins or publishing discovered skills to the model.
- Cache inventories by environment instance, executor session, working directory, and sandbox. Coalesce concurrent requests, refresh the cache at turn start, retry transient failures, and refresh MCP projections after recovery.
- Resolve the system skills root in the executor and pass it explicitly to capability location discovery.

## Testing

Add coverage for feature gating, unsupported executors, inert inventories, cache reuse and eviction, turn refreshes, failure recovery, executor replacement, and system skill locations.

GitOrigin-RevId: 1bdca28fa7f17622b27e2713ab3dcc0c13426889

codex-rs/exec-server-protocol/src/capabilities.rs

@@ -5,13 +5,21 @@ use codex_utils_path_uri::PathUri;
 use serde::Deserialize;
 use serde::Serialize;
 use std::fmt;
+use std::sync::Arc;
 
 use crate::CapabilityTextFile;
 use crate::DiscoveredPluginFiles;
 
 pub const CAPABILITIES_DISCOVER_V2_METHOD: &str = "capabilities/discoverV2";
 
-/// Discovers installed capabilities on the executor.
+/// Caller-bound discovery result shared by capability consumers within a model step.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct ExecutorCapabilityDiscoveryV2Snapshot {
+    pub environment_id: String,
+    pub result: Result<Arc<DiscoverV2CapabilitiesResponse>, String>,
+}
+
+/// Discover installed capabilities for a thread, without client-supplied installation paths.
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct DiscoverV2CapabilitiesRequest {