fix(ai): resolve Bedrock regions without implicit defaults

[rekram1-node]

Summary

• resolve native Bedrock Converse and Mantle regions from explicit region, existing credentials.region, then AWS_REGION; trim values and remove the silent us-east-1 fallback
• fail model initialization with an actionable typed AIError (InvalidRequest, parameter region) when region is missing or blank
• defer route construction until model selection so importing providers or creating an unconfigured facade remains safe
• allow region omission for custom baseURL plus bearer apiKey; SigV4 and generated AWS endpoints still require region
• use the resolved region for both endpoint construction and signing without mutating caller credentials
• resolve region before Core maps static credentials and substitutes ${AWS_REGION} in catalog URLs, including legacy endpoint aliases and directly selected native packages
• preserve native initialization AIErrors through ModelResolver and generation rather than replacing them with misleading unsupported-package errors

Scope

This is the focused native region/configuration fix. AWS profile/config-file discovery, credential chains and refresh, bearer-token environment resolution, and changes to the legacy AI SDK hooks are not included. No new public error schema or Server/Protocol API changes. Existing callers that relied on the native default region must now pass region or set AWS_REGION.

Also corrects one stale package-alias assertion left by #45394: Mantle defaults to Responses, not Chat.

Verification

• 65 targeted AI tests passed across region, Converse, Mantle, and native-provider suites
• provider package import/alias test passed with AWS_REGION unset
• 70 Core tests passed across native mapping, model resolution, generation, and session errors
• bun typecheck passed in packages/ai and packages/core
• Prettier and git diff --check passed
• independent review found and then verified the direct-native URL/signing-region fix; no remaining findings

Full-Suite Baseline

Full AI suite: 754 passed, 25 skipped, 14 failed. Clean v2 at 767115cd69: 748 passed, 25 skipped, 15 failed. No new failures; the Mantle alias assertion is fixed. The other failures are existing WebSocket executor expectations, Anthropic fixtures/package expectations, and OpenRouter fixtures/reasoning expectations. The full Core suite was not run.