Extensions calling ctx.ui.addAutocompleteProvider (e.g. @ff-labs/pi-fff)
crashed at load with 'TypeError: ... is not a function' because omp's
ExtensionAPI.ui omitted pi's autocomplete-provider API; the throw also
aborted the rest of a try/catch-guarded session_start init.
ExtensionUIContext now declares addAutocompleteProvider(factory).
Interactive mode stacks each factory on the built-in editor provider in
registration order, re-applies the stack on every slash-command refresh,
and skips throwing/malformed factories; RPC, ACP, and headless contexts
accept the factory as a no-op, matching upstream pi's RPC behavior.
Fixes#4919
Scheduled a TUI repaint after extension-driven prompt mutations so pasteToEditor and setEditorText do not leave the editor visually stale until the next input event.
Fixes#4341
can1357