fix(core): normalize provider endpoint templates

[kitlangton]

What

Resolve Bedrock Mantle catalog endpoints from the provider's configured region without teaching the generic model resolver about Bedrock.

This is the adapter-based alternative to #41786 and closes #40075.

Before / After

Before: models.dev supplies Mantle endpoints such as https://bedrock-mantle.${AWS_REGION}.api.aws/openai/v1. With settings.region: "us-west-2" and no AWS_REGION, ModelResolver rejected the intermediate catalog settings before the native Mantle adapter or provider could interpret them.

The regression test failed before the fix with:

Cannot initialize amazon-bedrock/test-model: AWS_REGION is required to resolve the provider endpoint

After: AISDKNative translates the known Mantle placeholder from the effective configured or credential region, preserving model-specific /v1 and /openai/v1 paths. Generic environment expansion and unresolved-variable validation then run on the effective mapped settings before provider loading. Explicit provider configuration therefore wins over the process environment while providers still construct routes from concrete URLs.

How

• packages/core/src/aisdk-native.ts resolves only the Mantle AWS_REGION placeholder while translating the AI SDK catalog package into the native AI package.
• packages/core/src/model-resolver.ts maps provider-specific settings first, then performs one-pass generic environment expansion and validation before loading or constructing the provider.
• Constructed routes receive a final unresolved-variable invariant check for provider-generated endpoints.
• Opaque third-party AI SDK packages use the same preparation before their loader runs because their wrapper route does not expose the provider's effective endpoint.
• packages/ai is unchanged; the native Mantle provider continues receiving a concrete baseURL and region.

Scope

• Generic ${ENV} behavior remains unchanged for other providers.
• Unknown unresolved variables still fail with UnresolvedProviderVariablesError.
• This does not add a generalized provider preparation hook or address unrelated models.dev templates.

Testing

• Before-fix reproduction: cd packages/core && bun run test test/model-resolver.test.ts failed with unresolved AWS_REGION.
• cd packages/core && bun run test test/model-resolver.test.ts test/aisdk-native.test.ts (47 pass)
• cd packages/core && bun typecheck
• Push hook: bun turbo typecheck --concurrency=3 (33 successful)

Flow

flowchart LR
  A[Catalog model and config] --> B[AI SDK native adapter]
  B -->|Mantle template plus region| C[Effective provider settings]
  C --> D[Generic environment expansion]
  D --> E{Variables unresolved?}
  E -->|Yes| F[UnresolvedProviderVariablesError]
  E -->|No| G[Provider constructs route]
  G --> H[Final endpoint invariant]
  H --> I[LanguageModel]

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