Add appearance metadata to thread sections (#37898)

## What changed

- Add optional `icon` and `color` appearance fields to custom thread sections and expose them through the app-server protocol.
- Persist appearance metadata in SQLite and include it when listing sections or returning threads.
- Let `threadSection/update` preserve an omitted appearance, clear it with `null`, or replace it with a new value.
- Reject appearance fields larger than 64 bytes.

## Testing

- Cover protocol compatibility, persistence across restart, update and clear behavior, and field-length validation.

GitOrigin-RevId: 8f55ce4f42f8bd17aa5416848a85845f681953df

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

@@ -176,6 +176,18 @@ pub struct ThreadSection {
     pub id: String,
     /// The current user-visible section name.
     pub name: String,
+    /// Optional appearance synchronized across clients.
+    #[serde(default)]
+    pub appearance: Option<ThreadSectionAppearance>,
+}
+
+/// Extensible visual presentation for a custom thread section.
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ThreadSectionAppearance {
+    pub icon: Option<String>,
+    pub color: Option<String>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS, ExperimentalApi)]