Harden validation error recovery

backend/src/main/java/fr/nessar/lmnporganizer/rental/adapter/in/web/common/RentalApiExceptionHandler.java

@@ -19,10 +19,14 @@ import org.slf4j.LoggerFactory;
 import org.springframework.dao.OptimisticLockingFailureException;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
+import org.springframework.http.converter.HttpMessageNotReadableException;
 import org.springframework.web.bind.MethodArgumentNotValidException;
+import org.springframework.web.bind.MissingServletRequestParameterException;
 import org.springframework.web.bind.annotation.ExceptionHandler;
 import org.springframework.web.bind.annotation.RestControllerAdvice;
+import org.springframework.web.method.annotation.MethodArgumentTypeMismatchException;
 import org.springframework.web.multipart.MaxUploadSizeExceededException;
+import org.springframework.web.multipart.support.MissingServletRequestPartException;
 
 @RestControllerAdvice(basePackages = "fr.nessar.lmnporganizer.rental.adapter.in.web")
 public class RentalApiExceptionHandler {
@@ -93,6 +97,27 @@ ResponseEntity<ValidationApiError> invalidRequest(MethodArgumentNotValidExceptio
 					.toList()));
 }
 
+@ExceptionHandler(HttpMessageNotReadableException.class)
+ResponseEntity<ValidationApiError> unreadableRequest() {
+	return validation("request", "malformed");
+}
+
+@ExceptionHandler(MethodArgumentTypeMismatchException.class)
+ResponseEntity<ValidationApiError> invalidParameter(MethodArgumentTypeMismatchException exception) {
+	return validation(exception.getName(), "invalid");
+}
+
+@ExceptionHandler(MissingServletRequestParameterException.class)
+ResponseEntity<ValidationApiError> missingParameter(
+	MissingServletRequestParameterException exception) {
+	return validation(exception.getParameterName(), "required");
+}
+
+@ExceptionHandler(MissingServletRequestPartException.class)
+ResponseEntity<ValidationApiError> missingPart(MissingServletRequestPartException exception) {
+	return validation(exception.getRequestPartName(), "required");
+}
+
 @ExceptionHandler(Exception.class)
 ResponseEntity<ApiError> unexpected(Exception exception) {
 	LOGGER.error("Unhandled rental API exception", exception);
@@ -107,6 +132,13 @@ private static ResponseEntity<ApiError> error(HttpStatus status, String message)
 	return ResponseEntity.status(status).body(new ApiError(message));
 }
 
+private static ResponseEntity<ValidationApiError> validation(String field, String code) {
+	return ResponseEntity.badRequest()
+		.body(
+			new ValidationApiError(
+				"validation_failed", java.util.List.of(new FieldValidationError(field, code))));
+}
+
 record ApiError(String message) {}
 
 record CodedApiError(String code, String message) {}

backend/src/test/java/fr/nessar/lmnporganizer/rental/adapter/in/web/RentalWebAdaptersTest.java

@@ -2,6 +2,7 @@ package fr.nessar.lmnporganizer.rental.adapter.in.web;
 
 import static org.hamcrest.Matchers.containsInAnyOrder;
 import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anyString;
 import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.Mockito.doThrow;
 import static org.mockito.Mockito.never;
@@ -31,6 +32,7 @@ import fr.nessar.lmnporganizer.rental.application.apartment.ApartmentCatalogServ
 import fr.nessar.lmnporganizer.rental.application.landlord.LandlordAccountService;
 import fr.nessar.lmnporganizer.rental.application.lease.AmendLeaseService;
 import fr.nessar.lmnporganizer.rental.application.lease.GenerateLeaseDocumentService;
+import fr.nessar.lmnporganizer.rental.application.lease.LeaseClauseCatalog;
 import fr.nessar.lmnporganizer.rental.application.lease.LeaseDraftService;
 import fr.nessar.lmnporganizer.rental.application.rent.AllocateRentPaymentService;
 import fr.nessar.lmnporganizer.rental.application.rent.GeneratePaymentReceiptService;
@@ -126,6 +128,7 @@ private static final RentPaymentRefundId REFUND_ID = new RentPaymentRefundId("re
 @MockitoBean private LeaseDraftService leaseDrafts;
 @MockitoBean private AmendLeaseService amendments;
 @MockitoBean private GenerateLeaseDocumentService leaseDocuments;
+@MockitoBean private LeaseClauseCatalog leaseClauseCatalog;
 @MockitoBean private RentPaymentHistoryService paymentHistory;
 @MockitoBean private RecordRentPaymentService paymentRecording;
 @MockitoBean private AllocateRentPaymentService paymentAllocation;
@@ -136,9 +139,17 @@ private static final RentPaymentRefundId REFUND_ID = new RentPaymentRefundId("re
 
 @BeforeEach
 void authenticateLandlordAndFixTime() {
+	Instant now = Instant.parse("2026-03-01T12:00:00Z");
 	when(accounts.identify(any())).thenReturn(landlord());
-	when(clock.instant()).thenReturn(Instant.parse("2026-03-01T12:00:00Z"));
+	when(clock.instant()).thenReturn(now);
 	when(clock.getZone()).thenReturn(ZoneOffset.UTC);
+	when(clock.withZone(any())).thenReturn(Clock.fixed(now, ZoneOffset.UTC));
+	LeaseClauseCatalog clauses =
+		LeaseClauseCatalog.load(getClass().getResourceAsStream("/lease-clauses.yml"));
+	when(leaseClauseCatalog.available(anyString(), any()))
+		.thenAnswer(
+			invocation ->
+				clauses.available(invocation.getArgument(0), invocation.getArgument(1)));
 }
 
 @Test
@@ -161,6 +172,25 @@ void protectedRentalMutationsRejectMissingCsrfTokens() throws Exception {
 		.andExpect(status().isForbidden());
 }
 
+@Test
+void malformedJsonIsAStableClientValidationFailure() throws Exception {
+	mockMvc
+		.perform(
+			post("/api/apartments")
+				.with(asLandlord())
+				.with(csrf())
+				.contentType(MediaType.APPLICATION_JSON)
+				.content(
+					"""
+					{"name":"Studio","address":"12 rue A","purchasePrice":"not-a-number",
+					"description":"","privateAreas":[]}
+					"""))
+		.andExpect(status().isBadRequest())
+		.andExpect(jsonPath("$.code").value("validation_failed"))
+		.andExpect(jsonPath("$.errors[0].field").value("request"))
+		.andExpect(jsonPath("$.errors[0].code").value("malformed"));
+}
+
 @Test
 void oidcClaimsScopeApartmentCreationAndLookupAndResponsesAreMapped() throws Exception {
 	Apartment apartment =
@@ -413,7 +443,11 @@ void leaseDraftRequestsRejectAnImpossibleAnnualRevisionDay() throws Exception {
 																	"referenceYear":2026,"seriesId":"001515333"}}}
 																"""))
 		.andExpect(status().isBadRequest())
-		.andExpect(jsonPath("$.message").value("Le jour annuel de révision IRL est invalide."));
+		.andExpect(jsonPath("$.code").value("validation_failed"))
+		.andExpect(
+			jsonPath("$.errors[0].field")
+				.value("legalTerms.rentRevision.annualRevisionDay"))
+		.andExpect(jsonPath("$.errors[0].code").value("invalid_date"));
 }
 
 @Test

backend/src/test/java/fr/nessar/lmnporganizer/rental/adapter/in/web/lease/LeaseAmendmentWebTest.java

@@ -16,6 +16,7 @@ import fr.nessar.lmnporganizer.rental.adapter.in.web.common.RentalApiExceptionHa
 import fr.nessar.lmnporganizer.rental.application.landlord.LandlordAccountService;
 import fr.nessar.lmnporganizer.rental.application.lease.AmendLeaseService;
 import fr.nessar.lmnporganizer.rental.application.lease.GenerateLeaseDocumentService;
+import fr.nessar.lmnporganizer.rental.application.lease.LeaseClauseCatalog;
 import fr.nessar.lmnporganizer.rental.application.lease.LeaseDraftService;
 import fr.nessar.lmnporganizer.rental.domain.landlord.ExternalIdentity;
 import fr.nessar.lmnporganizer.rental.domain.landlord.Landlord;
@@ -43,6 +44,7 @@ class LeaseAmendmentWebTest {
 @MockitoBean LeaseDraftService drafts;
 @MockitoBean AmendLeaseService amendments;
 @MockitoBean GenerateLeaseDocumentService documents;
+@MockitoBean LeaseClauseCatalog clauseCatalog;
 @MockitoBean Clock clock;
 
 @BeforeEach