fix(core): route Bedrock packages natively

[opencode-agent]

Summary

• map @ai-sdk/amazon-bedrock catalog entries to the native Bedrock Converse provider
• always map @ai-sdk/amazon-bedrock/mantle entries to the native Mantle Chat or Responses provider
• remove credential availability as a condition for native Mantle routing
• preserve serializable Bedrock provider settings: endpoint, auth, region, static credentials, headers, and topP
• project Converse request options into the native request body, including additional model fields, reasoning configuration, Anthropic betas, and service tier
• preserve supported OpenAI request options for Mantle

Runtime-only AI SDK hooks such as custom fetch and credentialProvider functions are intentionally not projected into serializable native provider settings.

This change does not expand regional URL templates; that is left for a separate change.

Verification

• cd packages/core && bun typecheck
• cd packages/core && bun test test/aisdk-native.test.ts test/model-resolver.test.ts

Requested by: @rekram1-node (Aiden Cline via Slack)