Add Bedrock Mantle GovCloud region

[CHARLESPALEN-OAI]

Summary

• Add us-gov-west-1 to the Bedrock Mantle supported region list
• Cover the GovCloud endpoint URL in the existing base_url unit test

Test

• cargo test -p codex-model-provider

[github-actions]

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[CHARLESPALEN-OAI]

I have read the CLA Document and I hereby sign the CLA

[chatgpt-codex-connector]

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[CHARLESPALEN-OAI]

recheck

[usgenes39-dotcom]

I found one blocking provider-routing issue on the current head.

The GovCloud region itself is valid, but the base URL path this PR codifies is not. AWS documents the Mantle OpenAI-compatible base URL as https://bedrock-mantle.<region>.api.aws/v1, and a no-auth probe returns 401 for /v1/models but 404 for /openai/v1/models in both us-gov-west-1 and us-east-1. With this PR, users who select the newly supported GovCloud region will route Responses requests to /openai/v1/responses, so the region unlock still fails at request time. CI is green, but this should be fixed before merge.