Add integration tests for validation, error handling, and task filtering

Introduced `ValidationAndErrorHandlingIT` and `PagingAndFilteringIT` integration tests to verify validations, error responses, and task filtering/pagination behaviors. Updated IntelliJ HTTP client script for task-related operations. Enhanced `ProjectTaskControllerIT` assertions for better coverage.

src/test/http/hemhub-api.http

@@ -113,6 +113,37 @@ Authorization: Bearer {{token}}
 GET http://localhost:8080/api/v1/households/{{householdId}}/tasks
 Authorization: Bearer {{token}}
 
+### 15a) Prepare a {{tomorrow}} variable (YYYY-MM-DD)
+GET http://localhost:8080/public/info
+
+> {%
+    const d = new Date();
+    d.setDate(d.getDate() + 1);
+    const tomorrow = d.toISOString().slice(0,10);
+    client.global.set("tomorrow", tomorrow);
+%}
+
+### 15b) Capture latest household task id (if not already set)
+GET http://localhost:8080/api/v1/households/{{householdId}}/tasks?size=1&sort=createdAt,desc
+Authorization: Bearer {{token}}
+
+> {%
+    if (!client.global.get("taskId")) {
+        const id = response.body?.content?.length ? response.body.content[0].id : null;
+        if (id) client.global.set("taskId", id);
+    }
+%}
+
+### 15c) Set that task's dueDate to {{tomorrow}}
+PATCH http://localhost:8080/api/v1/tasks/{{taskId}}
+Content-Type: application/json
+Authorization: Bearer {{token}}
+
+{
+  "dueDate": "{{tomorrow}}"
+}
+
+
 ### 16) My tasks due tomorrow
 GET http://localhost:8080/api/v1/tasks/due/tomorrow
 Authorization: Bearer {{token}}