feat: Add comprehensive GTest suite for Lance writer

Implements comprehensive unit and integration tests for Lance FFI writer:

**Test Coverage:**
- Basic initialization and path handling
- Single and multiple batch writes
- Schema locking and validation
- Empty and null batch handling
- Nullable field support
- All major Arrow data types (int32/64, float32/64, utf8, date32, boolean)
- Proper cleanup and destructor handling

**Test Organization:**
- LanceWriterBasicTest: Initialization tests
- LanceWriterIntegrationTest: End-to-end dataset creation tests
- LanceWriterDataTypeTest: Data type support verification

**Configuration:**
- Tests are guarded by TPCH_ENABLE_LANCE and TPCH_BUILD_TESTS flags
- Only built when both flags are enabled
- Follows existing test patterns (Paimon writer tests)
- Uses CMake gtest_discover_tests for automatic test discovery

**Benefits:**
- Regression prevention for Lance writer functionality
- Validates critical operations (schema locking, batch streaming)
- Ensures null handling and error conditions work correctly
- Safe guard for future maintenance and refactoring

Addresses PR #5 review comment: "Large new Lance writer functionality has no automated tests"

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: tsafin

tests/CMakeLists.txt

@@ -78,5 +78,25 @@ if(TPCH_BUILD_TESTS)
         gtest_discover_tests(paimon_writer_test)
     endif()
 
+    # Lance writer tests (only if Lance FFI is enabled)
+    if(TPCH_ENABLE_LANCE)
+        add_executable(lance_writer_test
+            lance_writer_test.cpp
+        )
+
+        target_link_libraries(lance_writer_test
+            PRIVATE
+                tpch_core
+                GTest::gtest_main
+        )
+
+        target_include_directories(lance_writer_test
+            PRIVATE
+                ${CMAKE_CURRENT_SOURCE_DIR}/../include
+        )
+
+        gtest_discover_tests(lance_writer_test)
+    endif()
+
     message(STATUS "Test targets configured successfully")
 endif()