fix: Bedrock API key region fallback (#25171)

## Why

Users following the Amazon Bedrock API-key setup can export
`AWS_BEARER_TOKEN_BEDROCK` and `AWS_REGION`, but Codex's bearer-token
auth path only accepted `model_providers.amazon-bedrock.aws.region`.
That made the documented env-based setup fail with a missing-region
error even though the standard AWS region environment variable was
present.

## What Changed

- Updates Bedrock bearer-token region resolution to use
`model_providers.amazon-bedrock.aws.region` first, then fall back to
`AWS_REGION`, then `AWS_DEFAULT_REGION`.
- Updates the missing-region error to list all supported region sources.
- Adds focused coverage for config precedence, `AWS_REGION`,
`AWS_DEFAULT_REGION`, and the missing-region failure.

codex-rs/model-provider/src/amazon_bedrock/auth.rs

@@ -20,6 +20,8 @@ use super::mantle::aws_auth_config;
 use super::mantle::region_from_config;
 
 const AWS_BEARER_TOKEN_BEDROCK_ENV_VAR: &str = "AWS_BEARER_TOKEN_BEDROCK";
+const AWS_REGION_ENV_VAR: &str = "AWS_REGION";
+const AWS_DEFAULT_REGION_ENV_VAR: &str = "AWS_DEFAULT_REGION";
 
 pub(super) enum BedrockAuthMethod {
     EnvBearerToken { token: String, region: String },
@@ -29,8 +31,8 @@ pub(super) enum BedrockAuthMethod {
 pub(super) async fn resolve_auth_method(
     aws: &ModelProviderAwsAuthInfo,
 ) -> Result<BedrockAuthMethod> {
-    if let Some(token) = bearer_token_from_env() {
-        let region = bearer_token_region_from_config(aws)?;
+    if let Some(token) = non_empty_env_var_from(AWS_BEARER_TOKEN_BEDROCK_ENV_VAR, std::env::var) {
+        let region = bearer_token_region(aws, std::env::var)?;
         return Ok(BedrockAuthMethod::EnvBearerToken { token, region });
     }
 
@@ -56,21 +58,30 @@ pub(super) async fn resolve_provider_auth(
     }
 }
 
-fn bearer_token_from_env() -> Option<String> {
-    std::env::var(AWS_BEARER_TOKEN_BEDROCK_ENV_VAR)
+fn non_empty_env_var_from(
+    name: &'static str,
+    env_var: impl Fn(&'static str) -> std::result::Result<String, std::env::VarError>,
+) -> Option<String> {
+    env_var(name)
         .ok()
-        .map(|token| token.trim().to_string())
-        .filter(|token| !token.is_empty())
+        .map(|value| value.trim().to_string())
+        .filter(|value| !value.is_empty())
 }
 
-fn bearer_token_region_from_config(aws: &ModelProviderAwsAuthInfo) -> Result<String> {
-    region_from_config(aws).ok_or_else(|| {
-        CodexErr::Fatal(
-            "Amazon Bedrock bearer token auth requires \
-`model_providers.amazon-bedrock.aws.region`"
-                .to_string(),
-        )
-    })
+fn bearer_token_region(
+    aws: &ModelProviderAwsAuthInfo,
+    env_var: impl Fn(&'static str) -> std::result::Result<String, std::env::VarError> + Copy,
+) -> Result<String> {
+    region_from_config(aws)
+        .or_else(|| non_empty_env_var_from(AWS_REGION_ENV_VAR, env_var))
+        .or_else(|| non_empty_env_var_from(AWS_DEFAULT_REGION_ENV_VAR, env_var))
+        .ok_or_else(|| {
+            CodexErr::Fatal(
+                "Amazon Bedrock bearer token auth requires \
+`model_providers.amazon-bedrock.aws.region`, `AWS_REGION`, or `AWS_DEFAULT_REGION`"
+                    .to_string(),
+            )
+        })
 }
 
 fn aws_auth_error_to_codex_error(error: AwsAuthError) -> CodexErr {
@@ -147,13 +158,23 @@ mod tests {
 
     use super::*;
 
+    fn missing_env_var(_: &'static str) -> std::result::Result<String, std::env::VarError> {
+        Err(std::env::VarError::NotPresent)
+    }
+
     #[test]
-    fn bedrock_bearer_auth_uses_configured_region_and_header() {
+    fn bedrock_bearer_auth_prefers_configured_region_and_uses_header() {
         let token = "bedrock-api-key-test".to_string();
-        let region = bearer_token_region_from_config(&ModelProviderAwsAuthInfo {
-            profile: None,
-            region: Some(" us-west-2 ".to_string()),
-        })
+        let region = bearer_token_region(
+            &ModelProviderAwsAuthInfo {
+                profile: None,
+                region: Some(" us-west-2 ".to_string()),
+            },
+            |name| match name {
+                AWS_REGION_ENV_VAR => Ok("eu-west-1".to_string()),
+                _ => Err(std::env::VarError::NotPresent),
+            },
+        )
         .expect("configured region should resolve");
         let provider = BearerAuthProvider {
             token: Some(token),
@@ -173,18 +194,55 @@ mod tests {
         );
     }
 
+    #[test]
+    fn bedrock_bearer_auth_uses_aws_region_env() {
+        let region = bearer_token_region(
+            &ModelProviderAwsAuthInfo {
+                profile: None,
+                region: None,
+            },
+            |name| match name {
+                AWS_REGION_ENV_VAR => Ok(" eu-central-1 ".to_string()),
+                _ => Err(std::env::VarError::NotPresent),
+            },
+        )
+        .expect("AWS_REGION should resolve");
+
+        assert_eq!(region, "eu-central-1");
+    }
+
+    #[test]
+    fn bedrock_bearer_auth_uses_aws_default_region_env() {
+        let region = bearer_token_region(
+            &ModelProviderAwsAuthInfo {
+                profile: None,
+                region: None,
+            },
+            |name| match name {
+                AWS_DEFAULT_REGION_ENV_VAR => Ok("ap-northeast-1".to_string()),
+                _ => Err(std::env::VarError::NotPresent),
+            },
+        )
+        .expect("AWS_DEFAULT_REGION should resolve");
+
+        assert_eq!(region, "ap-northeast-1");
+    }
+
     #[test]
     fn bedrock_bearer_auth_rejects_missing_configured_region() {
-        let err = bearer_token_region_from_config(&ModelProviderAwsAuthInfo {
-            profile: None,
-            region: None,
-        })
+        let err = bearer_token_region(
+            &ModelProviderAwsAuthInfo {
+                profile: None,
+                region: None,
+            },
+            missing_env_var,
+        )
         .expect_err("missing region should fail");
 
         assert_eq!(
             err.to_string(),
             "Fatal error: Amazon Bedrock bearer token auth requires \
-`model_providers.amazon-bedrock.aws.region`"
+`model_providers.amazon-bedrock.aws.region`, `AWS_REGION`, or `AWS_DEFAULT_REGION`"
         );
     }