feat(app-server): add v2 deprecation notice (#8285)

Add a v2 event for deprecation notices so we can get rid of
`codex/event/deprecation_notice`.

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -539,6 +539,7 @@ server_notification_definitions! {
     ReasoningSummaryPartAdded => "item/reasoning/summaryPartAdded" (v2::ReasoningSummaryPartAddedNotification),
     ReasoningTextDelta => "item/reasoning/textDelta" (v2::ReasoningTextDeltaNotification),
     ContextCompacted => "thread/compacted" (v2::ContextCompactedNotification),
+    DeprecationNotice => "deprecationNotice" (v2::DeprecationNoticeNotification),
 
     /// Notifies the user of world-writable directories on Windows, which cannot be protected by the sandbox.
     WindowsWorldWritableWarning => "windows/worldWritableWarning" (v2::WindowsWorldWritableWarningNotification),

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

@@ -1893,6 +1893,16 @@ pub struct AccountLoginCompletedNotification {
     pub error: Option<String>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct DeprecationNoticeNotification {
+    /// Concise summary of what is deprecated.
+    pub summary: String,
+    /// Optional extra guidance, such as migration steps or rationale.
+    pub details: Option<String>,
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;