[BUG]: Bedrock Mantle models unreachable on v2 — ${AWS_REGION} in base URL is never substituted

Description

On the v2 path, openai.* Bedrock models (GPT-5.6 Sol/Terra/Luna, gpt-oss) can never connect. The catalog advertises the Mantle endpoint as a template, https://bedrock-mantle.${AWS_REGION}.api.aws/openai/v1, and nothing substitutes it, so the request goes to that literal hostname:

Cannot connect to API: Unable to connect. Is the computer able to access the url?
Cannot connect to API: Was there a typo in the url or port?

v1 is fine — packages/opencode/src/provider/provider.ts expands it via the bedrock loader's vars() hook (added in #31001), and provider.ts:1712 does the replacement. The v2 plugin, packages/core/src/plugin/provider/amazon-bedrock.ts, resolves a region for the SDK but leaves options.baseURL untouched; packages/core/src/aisdk.ts:80 assigns model.api.url verbatim. Other v2 plugins expand their own templates (google-vertex.ts:78, cloudflare-workers-ai.ts:64), bedrock just does not.

Same class of bug likely affects the other templated catalog URLs on v2 (${SNOWFLAKE_ACCOUNT}, ${DATABRICKS_HOST}, ${NEON_AI_GATEWAY_BASE_URL}) — none of those plugins expand either. Happy to keep this issue scoped to bedrock.

OpenCode version

opencode2 0.0.0-next-16420 (the v2 binary from @opencode-ai/cli). Also confirmed by reading dev at 1.18.11, where the failing test below was written.

Steps to reproduce

1. Configure amazon-bedrock with any working auth and "region": "us-east-1".
2. Select openai.gpt-5.6-sol on v2.
3. Send a prompt — it fails to connect. The resolved URL is https://bedrock-mantle.${AWS_REGION}.api.aws/openai/v1/responses.

Operating System

macOS 26.5.1