[FEATURE]: Amazon Bedrock — support a second profile/account and reach Mantle Responses API from a non-"amazon-bedrock" provider

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Describe the enhancement you want to request

Follow-up to #30993 / #31001 (thanks for the Mantle fix). Two related limits show up when using Bedrock Mantle across two AWS accounts, on opencode 1.18.8:

1. One profile per provider. The SSO/profile credential chain (fromNodeProviderChain({ profile })) is wired only into the loader keyed to the literal id amazon-bedrock (provider.ts#L294-L352). I run Claude on account A and the Mantle GPT-5.x models on account B. A second, differently-named provider can't resolve a profile — it falls through to generic handling with no credentialProvider, so it only works with static SigV4 keys.

2. Responses API only reachable via id amazon-bedrock. selectBedrockMantleLanguageModel (which picks .responses() vs .chat()) is dispatched only from that same loader (L368 → L162-L166). Under any other provider id, Mantle models default to .chat(), so Responses-only models (gpt-5.x) return 400 "does not support /v1/chat/completions". Chat-only Mantle models (qwen etc.) do work via a second provider with static keys.

Plugins can't bridge this — ProviderHook only exposes model discovery, not a custom SDK/credential provider (plugin/src/index.ts#L214-L217).

Possible directions (or tell me I'm wrong):

• allow a per-model/per-config profile under amazon-bedrock, and/or
• fire the Mantle responses/chat selector based on model.api.npm === "@ai-sdk/amazon-bedrock/mantle" regardless of provider id.

Docs: the providers page currently documents only the standard Bedrock options (region/profile/endpoint); the Mantle path (npm: "@ai-sdk/amazon-bedrock/mantle", /openai/v1 base URL, Responses routing) isn't documented anywhere. Worth adding once the above is settled.

If there's already a supported way to do multi-account Bedrock + Mantle Responses today, I'll gladly just use that.