Stop emitting item/fileChange/outputDelta output delta notifications (#20471)

## Why

`item/fileChange/outputDelta` text output was only the tool's summary or
error text and not used by client surfaces.

We keep `item/fileChange/outputDelta` in the app-server protocol as a
deprecated compatibility entry, but the server no longer emits it.

## What changed

- stop the `apply_patch` runtime from emitting `ExecCommandOutputDelta`
events
- simplify `item_event_to_server_notification` so command output deltas
always map to `item/commandExecution/outputDelta`
- remove the app-server bookkeeping that tried to detect whether an
output delta belonged to a file change
- mark `item/fileChange/outputDelta` as a deprecated legacy protocol
entry in the v2 types, schema, and README
- simplify the file-change approval tests so they only wait for
completion instead of expecting output-delta notifications

## Testing

- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-thread-manager-sample`
- `cargo test -p codex-app-server-protocol
protocol::event_mapping::tests::exec_command_output_delta_maps_to_command_execution_output_delta
-- --exact`
- `cargo test -p codex-app-server
turn_start_file_change_approval_accept_for_session_persists_v2 --
--exact` *(failed before the test assertions because the wiremock
`/responses` mock received 0 requests in setup)*

codex-rs/app-server/tests/suite/v2/turn_start.rs

@@ -24,7 +24,6 @@ use codex_app_server_protocol::CommandExecutionApprovalDecision;
 use codex_app_server_protocol::CommandExecutionRequestApprovalResponse;
 use codex_app_server_protocol::CommandExecutionStatus;
 use codex_app_server_protocol::FileChangeApprovalDecision;
-use codex_app_server_protocol::FileChangeOutputDeltaNotification;
 use codex_app_server_protocol::FileChangePatchUpdatedNotification;
 use codex_app_server_protocol::FileChangeRequestApprovalResponse;
 use codex_app_server_protocol::ItemCompletedNotification;
@@ -2198,9 +2197,8 @@ async fn turn_start_file_change_approval_v2() -> Result<()> {
     )
     .await?;
     let mut saw_resolved = false;
-    let mut output_delta: Option<FileChangeOutputDeltaNotification> = None;
     let mut completed_file_change: Option<ThreadItem> = None;
-    while !(output_delta.is_some() && completed_file_change.is_some()) {
+    while completed_file_change.is_none() {
         let message = timeout(DEFAULT_READ_TIMEOUT, mcp.read_next_message()).await??;
         let JSONRPCMessage::Notification(notification) = message else {
             continue;
@@ -2217,16 +2215,6 @@ async fn turn_start_file_change_approval_v2() -> Result<()> {
                 assert_eq!(resolved.request_id, resolved_request_id);
                 saw_resolved = true;
             }
-            "item/fileChange/outputDelta" => {
-                assert!(saw_resolved, "serverRequest/resolved should arrive first");
-                let notification: FileChangeOutputDeltaNotification = serde_json::from_value(
-                    notification
-                        .params
-                        .clone()
-                        .expect("item/fileChange/outputDelta params"),
-                )?;
-                output_delta = Some(notification);
-            }
             "item/completed" => {
                 let completed: ItemCompletedNotification = serde_json::from_value(
                     notification.params.clone().expect("item/completed params"),
@@ -2239,16 +2227,6 @@ async fn turn_start_file_change_approval_v2() -> Result<()> {
             _ => {}
         }
     }
-    let output_delta = output_delta.expect("file change output delta should be observed");
-    assert_eq!(output_delta.thread_id, thread.id);
-    assert_eq!(output_delta.turn_id, turn.id);
-    assert_eq!(output_delta.item_id, "patch-call");
-    assert!(
-        !output_delta.delta.is_empty(),
-        "expected delta to be non-empty, got: {}",
-        output_delta.delta
-    );
-
     let completed_file_change =
         completed_file_change.expect("file change completion should be observed");
     let ThreadItem::FileChange { ref id, status, .. } = completed_file_change else {
@@ -3000,11 +2978,6 @@ async fn turn_start_file_change_approval_accept_for_session_persists_v2() -> Res
     )
     .await?;
 
-    timeout(
-        DEFAULT_READ_TIMEOUT,
-        mcp.read_stream_until_notification_message("item/fileChange/outputDelta"),
-    )
-    .await??;
     timeout(
         DEFAULT_READ_TIMEOUT,
         mcp.read_stream_until_notification_message("item/completed"),
@@ -3058,11 +3031,6 @@ async fn turn_start_file_change_approval_accept_for_session_persists_v2() -> Res
 
     // If the server incorrectly emits FileChangeRequestApproval, the helper below will error
     // (it bails on unexpected JSONRPCMessage::Request), causing the test to fail.
-    timeout(
-        DEFAULT_READ_TIMEOUT,
-        mcp.read_stream_until_notification_message("item/fileChange/outputDelta"),
-    )
-    .await??;
     timeout(
         DEFAULT_READ_TIMEOUT,
         mcp.read_stream_until_notification_message("item/completed"),