Expose plugin eligibility metadata in app-server summaries (#35837)

## What changed

- Add nullable `disabledReason` and `eligiblePlanTypes` fields to v2
  `PluginSummary` responses and generated schemas.
- Preserve the remote catalog values across discovered, installed, and cached
  plugin summary paths while returning `null` for local plugins and older
  remote responses.
- Treat unrecognized disabled reasons as `unknown` for forward compatibility.

## Testing

- Cover protocol round trips, remote summary propagation, unknown disabled
  reasons, and app-server responses for admin-disabled and plan-ineligible
  plugins.

GitOrigin-RevId: 657a7ea6f838a6ff8bd1769ebcca0f3432684437

codex-rs/app-server-protocol/src/protocol/v2/plugin.rs

@@ -619,6 +619,17 @@ pub enum PluginAvailability {
     DisabledByAdmin,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "snake_case")]
+#[ts(export_to = "v2/", rename_all = "snake_case")]
+pub enum PluginDisabledReason {
+    DisabledByAdmin,
+    PlanNotEligible,
+    RequiredAppUnavailable,
+    #[serde(other)]
+    Unknown,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
@@ -646,6 +657,12 @@ pub struct PluginSummary {
     /// Availability state for installing and using the plugin.
     #[serde(default)]
     pub availability: PluginAvailability,
+    /// Why the remote plugin is unavailable, when provided by plugin-service.
+    #[serde(default)]
+    pub disabled_reason: Option<PluginDisabledReason>,
+    /// Raw plugin-service plan identifiers eligible to install the plugin.
+    #[serde(default)]
+    pub eligible_plan_types: Option<Vec<String>>,
     pub interface: Option<PluginInterface>,
     #[serde(default)]
     pub keywords: Vec<String>,