feat(tests): increase test coverage from 68% to 78%

Add comprehensive integration and API tests:
- Integration tests for SQLAlchemyPostRepository (34 tests)
- API tests for posts endpoints and error handlers (22 tests)
- Unit tests for PublishPostUseCase and ListPostsUseCase
- Unit tests for SessionTransactionManager

Also register generic exception handler in error_handler.py

All 167 tests pass, coverage now meets CI threshold of 70%

tests/unit/infrastructure/test_transaction_manager.py

@@ -0,0 +1,46 @@
+"""Tests for DI transaction manager.
+
+Tests SessionTransactionManager implementation.
+"""
+
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.infrastructure.di.transaction_manager import SessionTransactionManager
+
+
+@pytest.fixture
+def mock_session() -> MagicMock:
+    """Create mock async session."""
+    session = MagicMock(spec=AsyncSession)
+    session.commit = AsyncMock()
+    session.rollback = AsyncMock()
+    return session
+
+
+@pytest.fixture
+def transaction_manager(mock_session: MagicMock) -> SessionTransactionManager:
+    """Create transaction manager with mock session."""
+    return SessionTransactionManager(mock_session)
+
+
+class TestSessionTransactionManager:
+    """Test suite for SessionTransactionManager."""
+
+    async def test_commit(
+        self, transaction_manager: SessionTransactionManager, mock_session: MagicMock
+    ) -> None:
+        """Test commit calls session commit."""
+        await transaction_manager.commit()
+
+        mock_session.commit.assert_called_once()
+
+    async def test_rollback(
+        self, transaction_manager: SessionTransactionManager, mock_session: MagicMock
+    ) -> None:
+        """Test rollback calls session rollback."""
+        await transaction_manager.rollback()
+
+        mock_session.rollback.assert_called_once()