feat: route guardian review model selection through providers (#22258)

## Why

Guardian review selection was hard-coded in `core`, which worked for the
default OpenAI path but did not give provider implementations a way to
choose backend-specific reviewer model IDs. That matters for Amazon
Bedrock: guardian review should run through the Bedrock/Mantle provider
using Bedrock's `openai.gpt-5.4` model ID, instead of accidentally
selecting a reviewer model that implies the OpenAI backend.

## What Changed

- Added provider-owned approval review model selection via
`ModelProvider::approval_review_model_selection`.
- Moved the existing default selection policy into the provider
abstraction: prefer the requested reviewer model when it is available,
otherwise fall back to the active turn model, preferring `Low` reasoning
when supported.
- Added an Amazon Bedrock override that pins guardian review to
`openai.gpt-5.4` with `Low` reasoning.

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

@@ -70,6 +70,10 @@ impl std::error::Error for ProviderAccountError {}
 
 pub type ProviderAccountResult = std::result::Result<ProviderAccountState, ProviderAccountError>;
 
+/// Default model used for automatic approval review when a provider does not
+/// require a backend-specific model ID.
+pub const DEFAULT_APPROVAL_REVIEW_PREFERRED_MODEL: &str = "codex-auto-review";
+
 /// Runtime provider abstraction used by model execution.
 ///
 /// Implementations own provider-specific behavior for a model backend. The
@@ -85,6 +89,13 @@ pub trait ModelProvider: fmt::Debug + Send + Sync {
         ProviderCapabilities::default()
     }
 
+    /// Returns the preferred model used for automatic approval review.
+    ///
+    /// Providers that require backend-specific model IDs should override this.
+    fn approval_review_preferred_model(&self) -> &'static str {
+        DEFAULT_APPROVAL_REVIEW_PREFERRED_MODEL
+    }
+
     /// Returns whether requests made through this provider should include attestation.
     fn supports_attestation(&self) -> bool {
         false
@@ -350,6 +361,19 @@ mod tests {
         assert_eq!(provider.capabilities(), ProviderCapabilities::default());
     }
 
+    #[test]
+    fn configured_provider_uses_default_approval_review_preferred_model() {
+        let provider = create_model_provider(
+            ModelProviderInfo::create_openai_provider(/*base_url*/ None),
+            /*auth_manager*/ None,
+        );
+
+        assert_eq!(
+            provider.approval_review_preferred_model(),
+            DEFAULT_APPROVAL_REVIEW_PREFERRED_MODEL
+        );
+    }
+
     #[tokio::test]
     async fn configured_provider_runtime_base_url_uses_configured_base_url() {
         let provider = create_model_provider(