Support no-auth mode for amazon-bedrock provider

What variant of Codex are you using?

cli

What feature would you like to see?

Codex's amazon-bedrock provider appears to always resolve AWS SDK auth/region before use, which makes it incompatible with an enterprise Bedrock gateway that exposes an OpenAI-compatible endpoint and handles auth externally, as Codex fails before making the request:

Fatal error: failed to resolve Amazon Bedrock auth: AWS SDK config did not resolve a region

Using a custom provider avoids AWS auth, e.g.

```toml
model = "openai.gpt-5.5"
model_provider = "bedrock-gateway"

[model_providers.bedrock-gateway]
base_url = "https://example.internal/openai/v1/"
wire_api = "responses"

  but then Codex emits:

Model metadata for 'openai.gpt-5.5' not found. Defaulting to fallback metadata; this can degrade
performance and cause issues.

  The custom provider works for requests, but loses the Bedrock/OpenAI model metadata that the native amazon-
  bedrock provider provides.


  ### Desired behavior

  Allow the amazon-bedrock provider to skip AWS SDK auth when base_url points at a gateway that handles
  authentication externally.

### Why 
  Enterprises commonly place gateways in front of Bedrock for SSO, audit, egress control, and centralized
  auth. In that setup, Codex should be able to use Bedrock model metadata without requiring local AWS
  credentials.

### Additional information

_No response_