Add workspace messages app-server API (#29001)

## Summary

- Add backend-client types and fetch support for active workspace
messages.
- Add the app-server v2 `account/workspaceMessages/read` method,
generated schemas, and README documentation.
- Delegate workspace-message eligibility to the Codex backend feature
gate; map a backend 404 to `featureEnabled: false`.

## Testing

- `just write-app-server-schema`
- `just test -p codex-backend-client`
- `just test -p codex-app-server-protocol`
- `just test -p codex-app-server workspace_messages`
- `just fix -p codex-backend-client -p codex-app-server-protocol -p
codex-app-server`
- `just fmt`

## Stack

- Base PR for #28232, which adds the TUI status-line integration.

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

@@ -314,6 +314,41 @@ pub struct GetAccountTokenUsageResponse {
     pub daily_usage_buckets: Option<Vec<AccountTokenUsageDailyBucket>>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct GetWorkspaceMessagesResponse {
+    /// Whether the workspace-message backend route is available for this client.
+    pub feature_enabled: bool,
+    /// Active workspace messages returned by the backend.
+    pub messages: Vec<WorkspaceMessage>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct WorkspaceMessage {
+    pub message_id: String,
+    pub message_type: WorkspaceMessageType,
+    pub message_body: String,
+    /// Unix timestamp (in seconds) when the message was created.
+    #[ts(type = "number | null")]
+    pub created_at: Option<i64>,
+    /// Unix timestamp (in seconds) when the message was archived.
+    #[ts(type = "number | null")]
+    pub archived_at: Option<i64>,
+}
+
+#[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 WorkspaceMessageType {
+    Headline,
+    Announcement,
+    #[serde(other)]
+    Unknown,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]