Allow Foundation Models to use other models

[cyanzhong]

Be prepared to support "Private Cloud Compute" model. This will not be functional until we migrate to Xcode 27.

[Copilot]

Pull request overview

This PR updates the Foundation Models bridge (JS ↔ native Swift) to carry an explicit modelName so the editor can route requests to different underlying models in the future (e.g., “Private Cloud Compute”).

Changes:

• Extend the native Foundation Models protocol/bridge to accept modelName for availability checks and session creation.
• Update the CoreEditor TypeScript native module interface and languageModel() API to pass a selected model name through to native.
• Add native-side model selection plumbing (createModel(named:)) to map modelName to a SystemLanguageModel.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 5 comments.

File	Description
MarkEditKit/Sources/Bridge/Native/Modules/EditorModuleFoundationModels.swift	Adds modelName plumbing and model selection helper for macOS 26 FoundationModels usage.
MarkEditKit/Sources/Bridge/Native/Generated/NativeModuleFoundationModels.swift	Updates generated native bridge decoding/routing to pass modelName.
CoreEditor/src/bridge/native/foundationModels.ts	Updates TS bridge interface signatures to include modelName.
CoreEditor/src/api/languageModel.ts	Updates public languageModel() API to require a model name and forwards it to native.

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[cyanzhong]

@copilot Review from scratch since the implementation has been fundamentally changed.

[Copilot]

@copilot Review from scratch since the implementation has been fundamentally changed.

Reviewed the current implementation from scratch. Overall it looks sound — no correctness or security issues found. Notes:

Model routing is correct. In languageModel(named:), the default branch returns SystemLanguageModel.default, which is Apple's on‑device Foundation Models. Since markedit-api v0.27.0 only exports LanguageModelName = 'Apple-Foundation-Models', every valid call today lands on that branch and gets the correct model. The "Private-Cloud-Compute" case is forward-looking and currently unreachable until the API adds that name — just make sure the string literal here matches the future LanguageModelName value exactly when you migrate to Xcode 27.

macOS 26 (< 27) silently ignores modelName. In both createSession and availability, when only macOS 26 is available the code always uses the default model regardless of modelName. That's fine today (PCC requires macOS 27 and the only valid name maps to default anyway), but worth keeping in mind once a second selectable model exists — a request for a non‑default model would silently fall back to the default rather than report unavailable.

availability(of:) looks right — it returns PCC-specific availability when the model is PrivateCloudComputeLanguageModel and defaultModelAvailability otherwise.

The version gating (#if canImport(FoundationModels, _version: 2) + #available(macOS 27.0, *)) and the com.apple.developer.private-cloud-compute entitlement are consistent with the "not functional until Xcode 27" intent. No changes needed.

Note: automated review flagged the default → SystemLanguageModel.default path as a bug — that's a false positive, since that path correctly handles 'Apple-Foundation-Models'.