fix(api): surface real provider error on apply/check instead of generic internal error

resolve (shared by Check/Apply) and Apply now wrap GetRecords/ApplyChanges
failures in service.ErrProviderUnavailable, matching ZoneRecords' existing
behavior. handleApply/handleCheck use errors.Is against it to return 502
with the real provider message (e.g. Selectel's 409 conflict body) instead
of masking every failure as a generic 500 "internal error"; non-provider
errors (decrypt/db/loader) are unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BwxdSt4reTm7Dj1oxRvpP3

internal/api/api_test.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -26,6 +27,10 @@ type mockCheckApplier struct {
 	// used by handleCheck status-persistence tests (drift/in_sync/error).
 	checkCS  *diff.Changeset
 	checkErr error
+
+	// applyErr, when set, makes Apply fail with this error — used by the
+	// provider-error-surfacing tests (502 with real message vs 500 generic).
+	applyErr error
 }
 
 func (m *mockCheckApplier) Check(context.Context, uuid.UUID, uuid.UUID) (diff.Changeset, error) {
@@ -40,6 +45,9 @@ func (m *mockCheckApplier) Check(context.Context, uuid.UUID, uuid.UUID) (diff.Ch
 }
 func (m *mockCheckApplier) Apply(_ context.Context, _, _ uuid.UUID, req service.ApplyRequest) (diff.Changeset, error) {
 	m.lastReq = req
+	if m.applyErr != nil {
+		return diff.Changeset{}, m.applyErr
+	}
 	return diff.Changeset{}, nil
 }
 func (m *mockCheckApplier) ZoneRecords(context.Context, uuid.UUID, uuid.UUID) ([]model.Record, error) {
@@ -271,6 +279,99 @@ func TestCheckEndpoint_ErrorScopesStatusToCallerProject(t *testing.T) {
 	}
 }
 
+// TestApplyEndpoint_ProviderErrorSurfacesRealMessage covers the fix: when
+// Svc.Apply fails with an error wrapping service.ErrProviderUnavailable (e.g.
+// Selectel rejecting a change with a 409 conflict), the handler must respond
+// 502 with the actual provider message in body.error — not a generic 500
+// "internal error" that hides the real cause from the user.
+func TestApplyEndpoint_ProviderErrorSurfacesRealMessage(t *testing.T) {
+	a, m := newTestAPI()
+	m.applyErr = fmt.Errorf("%w: %v", service.ErrProviderUnavailable,
+		errors.New("selectel POST /zones/x/rrset: 409: conflicting CNAME record exists"))
+	router := NewRouter(a)
+
+	did := uuid.New().String()
+	req := requestWithSessionCookie(http.MethodPost,
+		"/api/v1/projects/00000000-0000-0000-0000-000000000002/domains/"+did+"/apply",
+		strings.NewReader(`{}`))
+	w := httptest.NewRecorder()
+	router.ServeHTTP(w, req)
+
+	if w.Code != http.StatusBadGateway {
+		t.Fatalf("expected 502, got %d body %s", w.Code, w.Body.String())
+	}
+	var resp map[string]string
+	if err := json.Unmarshal(w.Body.Bytes(), &resp); err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(resp["error"], "409") || !strings.Contains(resp["error"], "conflicting CNAME") {
+		t.Fatalf("expected real provider message in body, got %q", resp["error"])
+	}
+	if strings.Contains(resp["error"], "internal error") {
+		t.Fatalf("provider error must not be masked as internal error, got %q", resp["error"])
+	}
+}
+
+// TestApplyEndpoint_NonProviderErrorStaysGeneric covers the flip side: an
+// unwrapped/local error (decrypt, db, loader) from Svc.Apply must still fall
+// back to a generic 500 "internal error" — only provider errors get their
+// real message surfaced.
+func TestApplyEndpoint_NonProviderErrorStaysGeneric(t *testing.T) {
+	a, m := newTestAPI()
+	m.applyErr = errors.New("decrypt: cipher: message authentication failed")
+	router := NewRouter(a)
+
+	did := uuid.New().String()
+	req := requestWithSessionCookie(http.MethodPost,
+		"/api/v1/projects/00000000-0000-0000-0000-000000000002/domains/"+did+"/apply",
+		strings.NewReader(`{}`))
+	w := httptest.NewRecorder()
+	router.ServeHTTP(w, req)
+
+	if w.Code != http.StatusInternalServerError {
+		t.Fatalf("expected 500, got %d body %s", w.Code, w.Body.String())
+	}
+	var resp map[string]string
+	if err := json.Unmarshal(w.Body.Bytes(), &resp); err != nil {
+		t.Fatal(err)
+	}
+	if resp["error"] != "internal error" {
+		t.Fatalf("expected generic internal error, got %q", resp["error"])
+	}
+}
+
+// TestCheckEndpoint_ProviderErrorSurfacesRealMessage mirrors the Apply case
+// for /check: a provider-wrapped error must come back as 502 with the real
+// provider message, while the existing status-persistence behavior (SetDomainStatus
+// error before responding) is unaffected.
+func TestCheckEndpoint_ProviderErrorSurfacesRealMessage(t *testing.T) {
+	a, m := newTestAPI()
+	ts := a.Store.(*mockTenantStore)
+	m.checkErr = fmt.Errorf("%w: %v", service.ErrProviderUnavailable,
+		errors.New("selectel GET /zones/x/rrset: 503: upstream unavailable"))
+	router := NewRouter(a)
+
+	did := uuid.New()
+	req := requestWithSessionCookie(http.MethodGet,
+		"/api/v1/projects/00000000-0000-0000-0000-000000000002/domains/"+did.String()+"/check", nil)
+	w := httptest.NewRecorder()
+	router.ServeHTTP(w, req)
+
+	if w.Code != http.StatusBadGateway {
+		t.Fatalf("expected 502, got %d body %s", w.Code, w.Body.String())
+	}
+	var resp map[string]string
+	if err := json.Unmarshal(w.Body.Bytes(), &resp); err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(resp["error"], "503") || !strings.Contains(resp["error"], "upstream unavailable") {
+		t.Fatalf("expected real provider message in body, got %q", resp["error"])
+	}
+	if len(ts.statusCalls) != 1 || ts.statusCalls[0].status != service.StatusError {
+		t.Fatalf("expected SetDomainStatus(_, _, error) to still run, got %+v", ts.statusCalls)
+	}
+}
+
 // TestChangesetResponseEmptyMarshalsToArrays guards the белый-экран bug: an
 // empty changeset (zone matches its template exactly, e.g. right after a
 // snapshot) must marshal updates/prunes/readOnly as [] not null — a nil slice