chore: improve expired Bedrock credential errors (#28992)

## Why

Amazon Bedrock returns a `401 Unauthorized` response containing
`Signature expired:` when an AWS credential, including a short-lived
`AWS_BEARER_TOKEN_BEDROCK`, has expired. Codex currently surfaces that
response as a generic `unexpected status` error, which does not explain
how to recover.

Environment-provided bearer tokens cannot be refreshed automatically, so
the error should direct users to refresh their AWS credentials or
replace or remove the environment token and restart Codex. This
classification belongs to the Amazon Bedrock provider so similar
responses from other providers retain their existing behavior.

## What changed

- Add a synchronous `ModelProvider::map_api_error` hook that defaults to
the existing provider-neutral API error mapping, and route model
request, stream, WebSocket, and terminal unauthorized errors through the
active provider.
- Override the hook for Amazon Bedrock. After preserving the structured
status, body, URL, and request metadata, recognize `401` responses
containing `Signature expired:` and attach actionable credential
guidance.
- Keep `codex-protocol` provider-neutral by representing the guidance as
an optional `user_message`. Error rendering prefers this message while
continuing to append the URL, request ID, Cloudflare ray, and
authorization diagnostics.
- Add model-provider coverage for expired signatures and negative cases,
core coverage for provider dispatch after unauthorized recovery, and a
TUI snapshot for the rendered error.

## Testing
Tested with a real request with expired bedrock key:
<img width="962" height="126" alt="Screenshot 2026-06-22 at 3 56 51 PM"
src="https://github.com/user-attachments/assets/7e21cc7c-798e-4662-8467-7f304a2f2b59"
/>

codex-rs/core/src/client_tests.rs

@@ -15,10 +15,13 @@ use crate::test_support::TestCodexResponsesRequestKind;
 use crate::test_support::responses_metadata as test_responses_metadata;
 use codex_api::ApiError;
 use codex_api::ResponseEvent;
+use codex_api::TransportError;
 use codex_app_server_protocol::AuthMode;
 use codex_login::AuthManager;
 use codex_login::CodexAuth;
 use codex_model_provider::BearerAuthProvider;
+use codex_model_provider::SharedModelProvider;
+use codex_model_provider::create_model_provider;
 use codex_model_provider_info::CHATGPT_CODEX_BASE_URL;
 use codex_model_provider_info::ModelProviderInfo;
 use codex_model_provider_info::WireApi;
@@ -81,6 +84,10 @@ fn test_model_client(session_source: SessionSource) -> ModelClient {
     )
 }
 
+fn test_model_provider() -> SharedModelProvider {
+    test_model_client(SessionSource::Cli).state.provider.clone()
+}
+
 fn test_responses_metadata_for_client(
     client: &ModelClient,
     turn_id: Option<&str>,
@@ -391,6 +398,7 @@ async fn dropped_response_stream_traces_cancelled_partial_output() -> anyhow::Re
         api_stream,
         test_session_telemetry(),
         attempt,
+        test_model_provider(),
     );
 
     let observed = stream
@@ -440,6 +448,7 @@ async fn response_stream_records_last_model_feedback_ids() {
         api_stream,
         test_session_telemetry(),
         InferenceTraceAttempt::disabled(),
+        test_model_provider(),
     );
 
     while stream.next().await.is_some() {}
@@ -455,6 +464,39 @@ async fn response_stream_records_last_model_feedback_ids() {
     );
 }
 
+#[tokio::test]
+async fn bedrock_unauthorized_error_uses_provider_mapping() {
+    let provider = create_model_provider(
+        ModelProviderInfo::create_amazon_bedrock_provider(/*aws*/ None),
+        /*auth_manager*/ None,
+    );
+    let mut auth_recovery = None;
+    let url = "https://bedrock-mantle.us-east-2.api.aws/openai/v1/responses";
+    let error = super::handle_unauthorized(
+        TransportError::Http {
+            status: http::StatusCode::UNAUTHORIZED,
+            url: Some(url.to_string()),
+            headers: None,
+            body: Some(
+                "Signature expired: 20260609T133205Z is now earlier than 20260614T062525Z"
+                    .to_string(),
+            ),
+        },
+        &mut auth_recovery,
+        &test_session_telemetry(),
+        &provider,
+    )
+    .await
+    .expect_err("expired Bedrock signature should fail");
+
+    assert_eq!(
+        error.to_string(),
+        format!(
+            "Amazon Bedrock rejected the request because its AWS signature has expired. Refresh your AWS credentials and retry. If `AWS_BEARER_TOKEN_BEDROCK` is set, update or unset it, then restart Codex, url: {url}"
+        )
+    );
+}
+
 #[tokio::test]
 async fn dropped_backpressured_response_stream_traces_cancelled_partial_output()
 -> anyhow::Result<()> {
@@ -481,6 +523,7 @@ async fn dropped_backpressured_response_stream_traces_cancelled_partial_output()
         api_stream,
         test_session_telemetry(),
         attempt,
+        test_model_provider(),
     );
 
     // Fill the mapper channel with non-terminal events, then yield one output