chore(app-server): stop emitting codex/event/ notifications (#14392)

## Description

This PR stops emitting legacy `codex/event/*` notifications from the
public app-server transports.

It's been a long time coming! app-server was still producing a raw
notification stream from core, alongside the typed app-server
notifications and server requests, for compatibility reasons. Now,
external clients should no longer be depending on those legacy
notifications, so this change removes them from the stdio and websocket
contract and updates the surrounding docs, examples, and tests to match.

### Caveat
I left the "in-process" version of app-server alone for now, since
`codex exec` was recently based on top of app-server via this in-process
form here: https://github.com/openai/codex/pull/14005

Seems like `codex exec` still consumes some legacy notifications
internally, so this branch only removes `codex/event/*` from app-server
over stdio and websockets.

## Follow-up

Once `codex exec` is fully migrated off `codex/event/*` notifications,
we'll be able to stop emitting them entirely entirely instead of just
filtering it at the external transport boundary.

codex-rs/app-server-protocol/schema/json/v1/InitializeParams.json

@@ -31,7 +31,7 @@
           "type": "boolean"
         },
         "optOutNotificationMethods": {
-          "description": "Exact notification method names that should be suppressed for this connection (for example `codex/event/session_configured`).",
+          "description": "Exact notification method names that should be suppressed for this connection (for example `thread/started`).",
           "items": {
             "type": "string"
           },