fix(core): resolve Bedrock region templates

[kitlangton]

What

Resolve Bedrock Mantle catalog endpoints containing ${AWS_REGION} from the provider's configured settings.region, even when AWS_REGION is absent from the process environment.

Closes #40075

Before / After

Before: A Bedrock config with settings.region: "us-west-2" and the catalog URL https://bedrock-mantle.${AWS_REGION}.api.aws/openai/v1 failed during model resolution when AWS_REGION was unset. The generic unresolved-variable guard raised UnresolvedProviderVariablesError before the merged native Mantle route could construct or use its regional endpoint.

After: The known Bedrock Mantle AWS_REGION placeholder resolves from configured settings.region first. The native Responses/Chat provider receives https://bedrock-mantle.us-west-2.api.aws/openai/v1, with its endpoint and signing region aligned.

How

• packages/core/src/model-resolver.ts recognizes only the Bedrock Mantle package and only the AWS_REGION placeholder, preferring configured region before the existing environment substitution pass.
• packages/core/test/model-resolver.test.ts covers the integrated catalog-to-native-provider path with configured region, no AWS_REGION, and the literal catalog template.
• The regression failed before the fix with AWS_REGION is required to resolve the provider endpoint.

Scope

• Generic environment-template substitution semantics are unchanged for other packages and placeholders.
• Native Mantle routing remains in place; this does not revive the obsolete provider-plugin seam used by fix(core): expand AWS_REGION in Bedrock Mantle base URL #40076.
• Credential provider callbacks are not projected into native serialized settings.
• This does not address the other templated catalog URLs mentioned in [BUG]: Bedrock Mantle models unreachable on v2 — ${AWS_REGION} in base URL is never substituted #40075.

Testing

• cd packages/core && bun run test test/model-resolver.test.ts test/aisdk-native.test.ts (42 pass)
• cd packages/core && bun typecheck
• cd packages/ai && bun typecheck
• Push hook: bun turbo typecheck --concurrency=3 (33 tasks pass)
• cd packages/ai && bun run test test/provider/bedrock-mantle.test.ts test/provider/bedrock-converse.test.ts test/provider-package.test.ts (60 pass, 1 pre-existing failure in the unchanged supports bearer authentication and custom base URLs test, which receives an empty recorded response)
• cd packages/core && bunx prettier --check src/model-resolver.ts test/model-resolver.test.ts

[kitlangton]

This was generated by AI during triage.

Closing in favor of #41816. The replacement keeps Bedrock-specific endpoint normalization in the existing AI SDK-to-native adapter, then performs generic expansion and validation before provider construction, rather than adding Bedrock knowledge to the generic model resolver.