Strip the API key from fixed error text and keep typed matches

ProviderError and the new NewRateLimitedError constructor now keep the
credential and remove it from their complete Error() text, including
fixed words such as "HTTP" and "rate", so errors.As to them succeeds
through the adapter's redaction stand-in instead of being refused. The
openai adapter builds RateLimitedError with the key. Tests cover the keys
"HTTP" (500), "rate" (429) and "openai" (401 and no model), checking
Error(), Body(), every unwrap layer, errors.Is and errors.As.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/ai/llm/errors_test.go

@@ -159,3 +159,28 @@ func TestProviderErrorTruncationMarkCannotCompleteSecret(t *testing.T) {
 		t.Fatalf("secret leaked: %q", pe.Body())
 	}
 }
+
+func TestProviderErrorWithFixedWordSecretNeverLeaksSecret(t *testing.T) {
+	const secret = "HTTP"
+	pe := NewProviderError(500, []byte(`{"error":"HTTP down"}`), secret)
+	if strings.Contains(pe.Error(), secret) || strings.Contains(pe.Body(), secret) {
+		t.Fatalf("secret leaked: Error %q, Body %q", pe.Error(), pe.Body())
+	}
+	if got := NewProviderError(500, nil, secret).Error(); got != "llm: provider error: [REDACTED] 500" {
+		t.Fatalf("Error() without body = %q", got)
+	}
+}
+
+func TestRateLimitedErrorWithFixedWordSecretNeverLeaksSecret(t *testing.T) {
+	for _, secret := range []string{"rate", "retry after"} {
+		for _, wait := range []time.Duration{0, 30 * time.Second} {
+			rl := NewRateLimitedError(wait, secret)
+			if strings.Contains(rl.Error(), secret) {
+				t.Fatalf("secret %q leaked: %q", secret, rl.Error())
+			}
+			if rl.RetryAfter != wait || !errors.Is(rl, ErrRateLimited) {
+				t.Fatalf("NewRateLimitedError(%v) = %+v", wait, rl)
+			}
+		}
+	}
+}