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/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -5099,6 +5099,7 @@
     },
     "FileChangeOutputDeltaNotification": {
       "$schema": "http://json-schema.org/draft-07/schema#",
+      "description": "Deprecated legacy notification for `apply_patch` textual output.\n\nThe server no longer emits this notification.",
       "properties": {
         "delta": {
           "type": "string"
@@ -11307,6 +11308,7 @@
           "type": "object"
         },
         {
+          "description": "Deprecated legacy apply_patch output stream notification.",
           "properties": {
             "method": {
               "enum": [