fix: Bedrock routing for OpenAI-compatible endpoints

[paylm]

Problem

Bedrock accounts can be selected for Anthropic-platform groups, but OpenAI-compatible routes such as /v1/chat/completions and /v1/responses were forwarding through the generic Anthropic upstream builder.

That path does not use Bedrock Runtime signing or API-key handling. When a Bedrock account is selected on /v1/chat/completions, the request can be sent as if it were a normal Anthropic API request, causing upstream authentication failures such as:

Authentication failed (401): x-api-key header is required

This is especially visible after the platform-based routing split for /v1/chat/completions, where non-OpenAI groups route to GatewayHandler.ChatCompletions and can schedule Bedrock accounts.

Fix

• Detect account.IsBedrock() inside GatewayService.ForwardAsChatCompletions and GatewayService.ForwardAsResponses.
• Route converted Anthropic request bodies through Bedrock Runtime request construction instead of the generic Anthropic upstream request builder.
• Preserve Bedrock SigV4 and Bedrock API Key modes:
  • SigV4 accounts use NewBedrockSignerFromAccount.
  • Bedrock API Key accounts use Authorization: Bearer <api_key>.
• Adapt Bedrock EventStream chunks back into Anthropic SSE so the existing Responses and Chat Completions conversion code can be reused.
• Skip normal Anthropic model normalization for Bedrock accounts and let the Bedrock resolver produce the AWS model id.

Tests

• Added coverage for Bedrock API Key accounts through /v1/chat/completions and /v1/responses, asserting requests go to bedrock-runtime with Authorization: Bearer and no x-api-key.
• Not run locally in this sandbox: no go/gofmt binary is available, and Docker daemon access is denied.

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[paylm]

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[paylm]

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