Enrich V2 capability discovery with resolved plugin and skill metadata (#53763)

## What changed

- Return qualified skill names such as `team:deploy` and parsed `dependencies` and `policy` instead of a separate namespace and raw metadata file. Share `agents/openai.yaml` parsing and validation between skill loaders and executor discovery.
- Include each standalone skill's search-root ID and path in `discoveryRoot`, while accepting responses from older executors that omit it.
- Add plugin `displayName`, falling back to the manifest name or installation directory.
- Materialize inline MCP declarations in `mcpConfig` with the manifest as their source path, and report incomplete discovery when declared MCP or Apps configuration files have no returned contents.

## Testing

Add coverage for JSON round trips, omitted `discoveryRoot`, optional and invalid skill metadata, qualified names, display-name fallbacks, and inline MCP configuration. Verify standalone search roots survive cached discovery and sandboxed RPC responses.

GitOrigin-RevId: 4124972444ad7c2fb8c6418acbf9a589d797fad2

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

@@ -966,7 +966,8 @@ pub struct CapabilityRootsDiscoverResponse {
     pub roots: Vec<CapabilityRootDiscovery>,
 }
 
-/// Recognized UTF-8 capability file materialized by the exec-server.
+/// UTF-8 capability contents materialized by the exec-server.
+/// `path` identifies the source file; V2 inline MCP contents are extracted from its manifest.
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct CapabilityTextFile {
@@ -979,7 +980,9 @@ pub struct CapabilityTextFile {
 #[serde(rename_all = "camelCase")]
 pub struct DiscoveredPluginFiles {
     pub manifest: CapabilityTextFile,
-    /// File-backed MCP declarations, including the conventional `.mcp.json` fallback.
+    /// MCP declarations from a selected file, including the conventional `.mcp.json` fallback.
+    /// V2 also materializes inline declarations, using the manifest as the source path.
+    /// V1 leaves inline declarations only in `manifest`.
     #[serde(default)]
     pub mcp_config: Option<CapabilityTextFile>,
     /// File-backed connector declarations.