fix(errors): recover retryable/status from wrapped ProxyError

IsRetryable and GetStatusCode used a direct type assertion, so a ProxyError
wrapped with %w was never detected (status fell back to 500, retryable to
false). Use errors.As, which dereferences the existing Unwrap chain and
matches the file's own Unwrap method.

Test: Test_IsRetryable_GetStatusCode_Wrapped (wrapped error preserves both
flags).
Note: these helpers currently have no production callers (test-only
surface), so this is a correctness hardening of the error API.

errors.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"time"
 )
@@ -121,7 +122,8 @@ func IsRetryable(err error) bool {
 		return false
 	}
 
-	if proxyErr, ok := err.(*ProxyError); ok {
+	var proxyErr *ProxyError
+	if errors.As(err, &proxyErr) {
 		return proxyErr.Retryable
 	}
 
@@ -134,7 +136,8 @@ func GetStatusCode(err error) int {
 		return 200
 	}
 
-	if proxyErr, ok := err.(*ProxyError); ok {
+	var proxyErr *ProxyError
+	if errors.As(err, &proxyErr) {
 		return proxyErr.StatusCode
 	}
 

errors_test.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"errors"
+	"fmt"
 	"net/http"
 	"testing"
 
@@ -241,3 +242,21 @@ func TestProxyError_Retryable(t *testing.T) {
 		})
 	}
 }
+
+// Test_IsRetryable_GetStatusCode_Wrapped verifies that retryability and status
+// are correctly recovered from a ProxyError that has been wrapped (e.g. via
+// %w), using errors.As rather than a direct type assertion.
+func Test_IsRetryable_GetStatusCode_Wrapped(t *testing.T) {
+	base := NewProxyError(ErrCodeBackendError, "boom", 503, true)
+	wrapped := fmt.Errorf("request failed: %w", base)
+	assert.True(t, IsRetryable(wrapped), "retryable flag must survive wrapping")
+	assert.Equal(t, 503, GetStatusCode(wrapped), "status code must survive wrapping")
+
+	nonRetryable := NewProxyError(ErrCodeInvalidRequest, "bad", 400, false)
+	wrapped2 := fmt.Errorf("wrapped: %w", nonRetryable)
+	assert.False(t, IsRetryable(wrapped2))
+	assert.Equal(t, 400, GetStatusCode(wrapped2))
+
+	assert.False(t, IsRetryable(nil))
+	assert.Equal(t, 200, GetStatusCode(nil))
+}