fix(core): prompt-family.test.ts fails typecheck on main (Provider.ID brand) - blocks pre-push for every branch #2195

Finding

On current main (9f4e222016, merge of PR #2194), bun run typecheck in packages/core fails with 4 errors, all in the test added by that PR:

• test/prompt-family.test.ts(51,71) and (52,71) (plus 2 more in the same file): TS2345 — Argument of type string is not assignable to parameter of type string & Brand<"Provider.ID">.

Verified independently in two trees this shift (2026-11-09T00:36Z): the clean primary checkout at 9f4e222016 and a worktree with current main merged in both produce exactly these 4 errors; no other file in packages/core errors.

Why it matters

The husky pre-push hook runs turbo typecheck, so every push of a branch based on current main fails pre-push (hit this shift landing PR #2174 for #2150; push required --no-verify after verifying the errors are pre-existing on main and untouched by that PR). Until fixed, every fleet lane merging main inherits a red typecheck and cannot distinguish its own errors from these.

Acceptance criteria

• bun run typecheck in packages/core passes on main (fix the test to construct Provider.ID values the branded way, or adjust the helper signature — whichever matches the codebase convention).
• No other typecheck errors introduced.