test + docs: close 12 test gaps (~250 new tests) and expand testing guide to 34 parts

Implements all P0-P2 test gaps from docs/test-gap-prompt.md:
- Deployment service tests (20), target service tests (18), scheduler tests (8)
- Agent binary tests (48), CSR renewal tests (8), short-lived cert tests (7)
- Domain model tests (25), context cancellation tests (9), concurrency tests (7)
- Handler negative-path tests (23 across 5 files)
- Frontend error handling tests (86) and API client tests (7)

Expands testing-guide.md from 28 to 34 parts covering certificate export,
S/MIME/EKU, OCSP/DER CRL, body size limits, Apache/HAProxy connectors,
and sub-CA mode. Fixes stale profile count (4->5) and updates sign-off table.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/api/handler/est_handler_test.go

@@ -396,3 +396,49 @@ func TestASN1EncodeLength(t *testing.T) {
 		}
 	}
 }
+
+func TestESTCSRAttrs_ServiceError(t *testing.T) {
+	svc := &mockESTService{
+		CSRAttrsErr: errors.New("service error"),
+	}
+	h := NewESTHandler(svc)
+
+	req := httptest.NewRequest(http.MethodGet, "/.well-known/est/csrattrs", nil)
+	w := httptest.NewRecorder()
+	h.CSRAttrs(w, req)
+
+	if w.Code != http.StatusInternalServerError {
+		t.Errorf("expected 500, got %d", w.Code)
+	}
+}
+
+func TestESTSimpleReEnroll_ServiceError(t *testing.T) {
+	csrPEM := generateTestCSRPEM(t)
+	svc := &mockESTService{
+		EnrollErr: errors.New("renewal failed"),
+	}
+	h := NewESTHandler(svc)
+
+	req := httptest.NewRequest(http.MethodPost, "/.well-known/est/simplereenroll", strings.NewReader(csrPEM))
+	w := httptest.NewRecorder()
+	h.SimpleReEnroll(w, req)
+
+	if w.Code != http.StatusInternalServerError {
+		t.Errorf("expected 500, got %d", w.Code)
+	}
+}
+
+func TestESTCACerts_UnableToGetCerts(t *testing.T) {
+	svc := &mockESTService{
+		CACertErr: errors.New("CA unavailable"),
+	}
+	h := NewESTHandler(svc)
+
+	req := httptest.NewRequest(http.MethodGet, "/.well-known/est/cacerts", nil)
+	w := httptest.NewRecorder()
+	h.CACerts(w, req)
+
+	if w.Code != http.StatusInternalServerError {
+		t.Errorf("expected 500, got %d", w.Code)
+	}
+}