Remove standard form input from app-server docs (#38083)

## What changed

Remove `openai/standard-form-input` from the documented app-server capability
extensions and initialization example. Continue to document `openai/form` for
extended form support.

GitOrigin-RevId: e1f313bec99a7fdc3462c8002181a8cdc48ad558

codex-rs/app-server/README.md

@@ -93,15 +93,11 @@ extended forms, clients must handle the request envelope, including a fallback
 for unsupported field types. `mcpServerOpenaiFormElicitation: true` remains a
 legacy alias for declaring the `openai/form` extension.
 
-`openai/standard-form-input` indicates support for collecting and returning
-input from standard MCP forms.
-
 ```json
 {
   "capabilities": {
     "extensions": {
       "openai/form": {},
-      "openai/standard-form-input": {},
       "io.modelcontextprotocol/ui": {
         "mimeTypes": ["text/html;profile=mcp-app"]
       }