test(vector): add assertions and fix minor issues in test suite

[SekaiArendelle]

• Add includes and comprehensive assert() checks to all vector container tests, verifying size, capacity, element values, and state after operations (push_back, pop_back, erase, insert, assign, swap, clear, shrink_to_fit, etc.)
• Fix typo in back.cc: "The first character" -> "The last character"
• Fix missing trailing newlines in multiple files
• Enable previously disabled append_range.cc test (remove #if 0)
• Add state assertions to shared_ptr index tests for insert_index and erase_index operations