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 default struct initialization for thread sanitizer compatibility
- Add 28 unit tests for HeapTable covering insert, update, delete, scan, MVCC
- Ignore _deps/ build artifact directory

Author: poyrazK

tests/heap_table_tests.cpp

@@ -312,7 +312,7 @@ TEST_F(HeapTableTests, TupleIdToString) {
 // ============= TupleHeader Tests =============
 
 TEST_F(HeapTableTests, TupleHeaderDefaults) {
-    HeapTable::TupleHeader header;
+    HeapTable::TupleHeader header{};
     EXPECT_EQ(header.xmin, 0U);
     EXPECT_EQ(header.xmax, 0U);
 }
@@ -328,7 +328,7 @@ TEST_F(HeapTableTests, TupleHeaderWithValues) {
 // ============= PageHeader Tests =============
 
 TEST_F(HeapTableTests, PageHeaderDefaults) {
-    HeapTable::PageHeader header;
+    HeapTable::PageHeader header{};
     EXPECT_EQ(header.next_page, 0U);
     EXPECT_EQ(header.num_slots, 0U);
     EXPECT_EQ(header.free_space_offset, 0U);