fix(models): align GetDefaultModel's defaultOn test with AvailableModels

[245678000000]

Summary

The two places that read ProviderModel.defaultOn disagreed on what a missing field means:

File	Expression	defaultOn: undefined
handlers/models/byokModelBuilder.ts	model.defaultOn ?? false	off
services/core/AiService.ts (GetDefaultModel)	x.model.defaultOn !== false	on

So a model with no defaultOn — a hand-written providers.json, or a config predating the sidebar writing the flag — is reported to the client as off in the picker, yet GetDefaultModel would push it as the default model for Composer and Cmd+K. DEFAULT_PROVIDERS seeds no models, so every model comes from user config and can hit this.

Change

A single isModelDefaultOn() predicate next to the ProviderModel type, with all four reads routed through it. It keeps the builder's meaning (only an explicit true counts as on) because that is the one that actually shapes the AvailableModels response the picker renders.

Consequence worth calling out: when nothing is enabled, GetDefaultModel now returns {} instead of picking the first model. That is the documented "don't push, client keeps its previous selection" path already in the handler's contract — but if you'd rather have the opposite fix (undefined meaning on everywhere, changing the wire response), say so and I'll flip it.

GetDefaultModel's body is lifted into a named handleGetDefaultModel, mirroring handleAvailableModels in the same file, so the behaviour is reachable from tests. It is exported for that reason.

Test plan

New Cursor++/src/server/tests/defaultModelSelection.test.ts — the predicate, the empty-candidate case, first-enabled/first-enabled-thinking selection, and an invariant that the pushed default is never a model AvailableModels reports as off. Three of the four fail on the old predicate (verified by reverting just the filter).

• pnpm --dir "Cursor++" run check-types
• pnpm --dir "Cursor++" run lint
• pnpm --dir "Cursor++" run test:server — 39 files / 371 tests passed (baseline on main is 38 / 367)

Those counts are from a machine with Cursor installed and a populated ~/.ccursor. On a clean checkout 5 of the 38 files fail for host-dependency reasons that have nothing to do with this change — 4 need @vscode/sqlite3 out of an installed Cursor, and protocol.test.ts:232 reads the real ~/.ccursor/knowledge-base.json. Detail in #20.