feat: Introduce comprehensive unit tests for models and BLoCs using mockito and bloc_test, and refine TripBloc error handling.

test/models/expense_test.dart

@@ -0,0 +1,91 @@
+import 'package:cloud_firestore/cloud_firestore.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:travel_mate/models/expense.dart';
+import 'package:travel_mate/models/expense_split.dart';
+
+void main() {
+  group('Expense Model Tests', () {
+    const id = 'expense1';
+    const groupId = 'group1';
+    const description = 'Lunch';
+    const amount = 50.0;
+    const currency = ExpenseCurrency.eur;
+    const amountInEur = 50.0;
+    const category = ExpenseCategory.restaurant;
+    const paidById = 'user1';
+    const paidByName = 'Alice';
+    final date = DateTime(2023, 6, 1);
+    final createdAt = DateTime(2023, 6, 1);
+    const split = ExpenseSplit(
+      userId: 'user1',
+      userName: 'Alice',
+      amount: 25.0,
+    );
+    const splits = [
+      split,
+      ExpenseSplit(userId: 'user2', userName: 'Bob', amount: 25.0),
+    ];
+
+    final expense = Expense(
+      id: id,
+      groupId: groupId,
+      description: description,
+      amount: amount,
+      currency: currency,
+      amountInEur: amountInEur,
+      category: category,
+      paidById: paidById,
+      paidByName: paidByName,
+      date: date,
+      createdAt: createdAt,
+      splits: splits,
+    );
+
+    test('supports value equality', () {
+      final expense2 = Expense(
+        id: id,
+        groupId: groupId,
+        description: description,
+        amount: amount,
+        currency: currency,
+        amountInEur: amountInEur,
+        category: category,
+        paidById: paidById,
+        paidByName: paidByName,
+        date: date,
+        createdAt: createdAt,
+        splits: splits,
+      );
+      expect(expense, equals(expense2));
+    });
+
+    group('fromMap', () {
+      test('parses correctly', () {
+        final map = {
+          'groupId': groupId,
+          'description': description,
+          'amount': amount,
+          'currency': 'EUR',
+          'amountInEur': amountInEur,
+          'category': 'restaurant', // matching category name
+          'paidById': paidById,
+          'paidByName': paidByName,
+          'date': Timestamp.fromDate(date),
+          'createdAt': Timestamp.fromDate(createdAt),
+          'splits': splits.map((s) => s.toMap()).toList(),
+        };
+
+        final fromMapExpense = Expense.fromMap(map, id);
+        expect(fromMapExpense, equals(expense));
+      });
+    });
+
+    group('copyWith', () {
+      test('updates fields correctly', () {
+        final updated = expense.copyWith(amount: 100.0);
+        expect(updated.amount, 100.0);
+        expect(updated.description, description);
+      });
+    });
+  });
+}