fix(ai): default native Mantle to Responses

[rekram1-node]

Summary

• make AmazonBedrockMantle.configure(...).model(...) and both default native package exports use Responses, matching the native OpenAI convention
• preserve explicit .chat(...) and .responses(...) methods and their separate native entrypoints/protocols
• retain HTTP-only Mantle Responses and the existing Core selection of Chat for the two safeguard models
• extend selection coverage to both safeguard IDs and verify the default exports resolve to Responses

Only the default aliases change in production code. Callers using the default entrypoint for Chat-only models must select Chat explicitly; Core already selects the explicit Chat entrypoint for openai.gpt-oss-safeguard-20b and openai.gpt-oss-safeguard-120b. No models.dev, authentication, metadata, or routing-abstraction changes.

Verification

• 13 AI tests passed across Mantle and native-provider suites
• 57 Core tests passed across native mapping and model resolution
• bun typecheck passed in packages/ai and packages/core
• Prettier checks passed for all four changed files
• git diff --check passed
• branch is up to date with v2 at 5b25ee8430