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-protocol/src/protocol/common.rs

@@ -1397,6 +1397,7 @@ server_notification_definitions! {
     CommandExecOutputDelta => "command/exec/outputDelta" (v2::CommandExecOutputDeltaNotification),
     CommandExecutionOutputDelta => "item/commandExecution/outputDelta" (v2::CommandExecutionOutputDeltaNotification),
     TerminalInteraction => "item/commandExecution/terminalInteraction" (v2::TerminalInteractionNotification),
+    /// Deprecated legacy apply_patch output stream notification.
     FileChangeOutputDelta => "item/fileChange/outputDelta" (v2::FileChangeOutputDeltaNotification),
     FileChangePatchUpdated => "item/fileChange/patchUpdated" (v2::FileChangePatchUpdatedNotification),
     ServerRequestResolved => "serverRequest/resolved" (v2::ServerRequestResolvedNotification),