fix(pi-fff): feature-detect ctx.ui.addAutocompleteProvider (#651)

[gustav-fff]

Closes #651

Root cause

packages/pi-fff/src/index.ts:411 calls ctx.ui.addAutocompleteProvider unconditionally inside session_start. The type at :406 also declares it as required. On pi forks that do not expose this method (omp, reported in #651), the call throws and the catch in session_start (:496-501) surfaces FFF init failed: ctx.ui.addAutocompleteProvider is not a function — and, more importantly, tool registration for the session never completes.

Fix

Mark addAutocompleteProvider as optional in the accepted ctx shape and feature-detect it before calling. When absent, skip the UI wiring and return — tools (find/grep/multi_grep) still register normally.

Steps to reproduce

Pre-fix, on origin/main:

git checkout origin/main
cd packages/pi-fff
bun install

Add a repro test that mimics an omp-style host lacking addAutocompleteProvider:

// packages/pi-fff/test/repro.test.ts
import { test, expect, mock } from "bun:test";
import fffExtension from "../src/index";

test("host without addAutocompleteProvider", async () => {
  const events = new Map<string, any>();
  const pi = {
    getFlag: mock(() => undefined),
    on: mock((e: string, h: any) => { events.set(e, h); }),
    registerCommand: mock(() => undefined),
    registerFlag: mock(() => undefined),
    registerTool: mock(() => undefined),
    appendEntry: mock(() => undefined),
  };
  const ctx = {
    cwd: "/tmp/workspace",
    ui: { notify: mock(() => undefined), setEditorComponent: mock(() => undefined) },
    // NOTE: no addAutocompleteProvider — matches omp fork surface
  };
  fffExtension(pi as any);
  await events.get("session_start")({ reason: "startup" }, ctx);
  expect(ctx.ui.notify).not.toHaveBeenCalled();
});

Run:

bun test test/repro.test.ts

Expected: pass.
Actual on origin/main: fails — ctx.ui.notify is called with FFF init failed: ctx.ui.addAutocompleteProvider is not a function, matching the traceback in the issue.

How verified

cd packages/pi-fff
bun test test/extension.test.ts

Output: 8 pass, 0 fail — includes the new session_start survives hosts without addAutocompleteProvider regression test plus the existing 7 tests unchanged.

Automated triage via Gustav. Honk-Honk 🪿