Add optional release IDs to cloud plugin source locations (#61393)

## What changed

Add `release_id: Option<String>` to `PluginSourceLocation::Cloud` to carry the opaque release ID selected by the cloud catalog for a plugin declaration. Update catalog and discovery test fixtures to supply the new field, including a populated release ID in catalog reconciliation tests.

GitOrigin-RevId: 1d60e25bd5fb8efd84e6e825305ad68b847fc1ee

codex-rs/core-plugins/src/catalog.rs

@@ -77,6 +77,8 @@ pub enum PluginSourceLocation {
     Cloud {
         resource_uri: String,
         bundle_uri: Option<String>,
+        /// Opaque release ID selected by the cloud catalog for this declaration.
+        release_id: Option<String>,
     },
     Executor {
         /// Executor environment that owns `root`.

codex-rs/core-plugins/src/catalog_tests.rs

@@ -25,6 +25,7 @@ fn cloud_plugin(remote_id: &str) -> PluginCatalogEntry {
         locations: vec![PluginSourceLocation::Cloud {
             resource_uri: format!("plugin://{remote_id}"),
             bundle_uri: None,
+            release_id: Some("release-1".to_string()),
         }],
     }
 }