feat: update Bedrock Mantle endpoint and GPT-5.4 model ID

[celia-oai]

Summary

Amazon Bedrock Mantle's OpenAI-compatible endpoint now lives under /openai/v1, and the GPT-5.4 Mantle model ID no longer uses the -cmb suffix. This updates Codex's built-in Bedrock provider configuration so generated providers and the static Bedrock catalog use the current endpoint and model ID.

Changes

• Update the Bedrock Mantle base URL from https://bedrock-mantle.{region}.api.aws/v1 to https://bedrock-mantle.{region}.api.aws/openai/v1.
• Update the Amazon Bedrock default base URL in codex-model-provider-info.
• Change the Bedrock GPT-5.4 catalog slug from openai.gpt-5.4-cmb to openai.gpt-5.4.
• Align provider and catalog tests with the new URL and model ID.

Test Plan

• Manual smoke test:

  target/debug/codex \
      -m openai.gpt-5.4 \
      -c 'model_provider="amazon-bedrock"' \
      -c 'model_providers.amazon-bedrock.aws.region="us-west-2"'