fix(heap_table): TupleId sentinel bug and add 28 unit tests

- Fix TupleId is_null() treating valid (0,0) as null (sentinel was 0, fixed to UINT32_MAX)
- Fix get_meta() not checking xmax, returning logically deleted tuples
- Fix fetch_page_by_id() returning zeroed page beyond EOF (infinite loop bug)
- Add 28 unit tests for HeapTable covering insert, update, delete, scan, MVCC
- Ignore _deps/ build artifact directory

Author: poyrazK

.gitignore

@@ -10,6 +10,7 @@ CMakeFiles/
 cmake_install.cmake
 Makefile
 *.cmake
+_deps/
 
 # Compiled binaries
 sqlEngine

CMakeLists.txt

@@ -135,6 +135,7 @@ if(BUILD_TESTS)
     add_cloudsql_test(raft_group_tests tests/raft_group_tests.cpp)
     add_cloudsql_test(raft_protocol_tests tests/raft_protocol_tests.cpp)
     add_cloudsql_test(columnar_table_tests tests/columnar_table_tests.cpp)
+    add_cloudsql_test(heap_table_tests tests/heap_table_tests.cpp)
     add_cloudsql_test(storage_manager_tests tests/storage_manager_tests.cpp)
     add_cloudsql_test(rpc_server_tests tests/rpc_server_tests.cpp)
     add_cloudsql_test(operator_tests tests/operator_tests.cpp)

include/storage/heap_table.hpp

@@ -37,11 +37,11 @@ class HeapTable {
         uint32_t page_num; /**< Physical page index in the file */
         uint16_t slot_num; /**< Logical slot index within the page */
 
-        TupleId() : page_num(0), slot_num(0) {}
+        TupleId() : page_num(UINT32_MAX), slot_num(0) {}
         TupleId(uint32_t page, uint16_t slot) : page_num(page), slot_num(slot) {}
 
         /** @return true if the ID represents a null/invalid record */
-        [[nodiscard]] bool is_null() const { return page_num == 0 && slot_num == 0; }
+        [[nodiscard]] bool is_null() const { return page_num == UINT32_MAX && slot_num == 0; }
 
         /** @return Human-readable string representation */
         [[nodiscard]] std::string to_string() const {

src/storage/heap_table.cpp

@@ -543,6 +543,12 @@ bool HeapTable::get_meta(const TupleId& tuple_id, TupleMeta& out_meta) const {
         std::memcpy(&out_meta.xmin, data + 2, 8);
         std::memcpy(&out_meta.xmax, data + 10, 8);
 
+        /* Skip tuples that have been logically deleted */
+        if (out_meta.xmax != 0) {
+            bpm_.unpin_page_by_id(file_id_, tuple_id.page_num, false);
+            return false;
+        }
+
         size_t cursor = 18;
         std::vector<common::Value> values;
         values.reserve(schema_.column_count());