refactor(decopilot): replace ban-types Function with a real call signature

[pedrofrxncx]

Source: C-DEBT — a ban-types-violating Function cast in studio-provider.ts (this repo's no-explicit-any/ban-types rules are warn-level, not CI-enforced, so this kind of thing accumulates uncaught). Distinct file from #6671 (which covers built-in-tools/index.ts and run-code.ts) — not a duplicate.

What/why: createLanguageModel's make() helper calls provider.aiSdk.languageModel with a provider-specific second settings argument that isn't part of the declared ProviderV3 interface (which only takes (modelId: string)). It reached for the banned Function type to bypass this, wrapped in a biome-ignore suppression comment. Replaced the cast with the real call signature (id: string, settings: Record<string, unknown>) => LanguageModelV3 — same runtime behavior, but now a signature drift on the underlying provider is a compile-time error instead of silently swallowed by Function, and the suppression comment is gone.

Confirms behavior is unchanged: pure type-level change — the call site and its two branches (with/without settings) are untouched, only the cast's declared type changed. bunx tsc --noEmit in apps/api is green on the file, no runtime logic touched.

Reviewer check: cd apps/api && bunx tsc --noEmit (green), bunx oxlint apps/api/src/harnesses/lib/decopilot/studio-provider.ts (0 warnings/errors).

Locally verified: bun run fmt, targeted tsc --noEmit in apps/api, oxlint on the changed file. Full CI validates the rest.

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Summary by cubic

Replaces the banned Function cast in studio-provider.ts with the real call signature (id: string, settings: Record<string, unknown>) => LanguageModelV3, so provider signature drift is caught at compile time instead of being silently swallowed.

This is a pure type-level refactor—the call site and runtime behavior are unchanged.

^{Written for commit 5f5cbde. Summary will update on new commits.}