Fix buffer overflow and memory leak in set method

[Copilot]

The set method had a buffer overflow writing one byte beyond allocated memory and a memory leak using the wrong delete operator for arrays.

Changes

Buffer overflow fix (line 126)

// Before: copied key_len + 1 bytes into key_len buffer
std::copy(key, key + key_len + 1, root_->prefix_);

// After: copy exactly key_len bytes
std::copy(key, key + key_len, root_->prefix_);

Memory leak fix (line 200)

// Before: wrong delete operator for array
delete old_prefix;

// After: use delete[] for arrays allocated with new[]
delete[] old_prefix;

Note on reported "aa\0" behavior

The issue description expected "aa" and "aa\0" to be distinct keys, but they're identical from the API's perspective:

art.set("aa", 3);    // strlen("aa") = 2, key_len = 3
art.set("aa\0", 10); // strlen("aa\0") = 2, key_len = 3 (stops at first null)
assert(art.get("aa") == 10); // Same key, value replaced

This is expected C-string semantics. The string literal "aa\0" contains ['a','a','\0','\0'], but strlen() stops at the first null. Supporting embedded nulls would require accepting explicit lengths, a breaking API change.

Added test case documenting this behavior.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Set key bug</issue_title>
<issue_description>```
art::art art;
art.set("aaa", 1);
art.set("aab", 2);
art.set("aa", 3);
art.set("aa\0", 10);
assert(art.get("aa") == 3);

This program will crush because of `assert(art.get("aa") == 3);` and the value of `aa` is  10.
While insert 'aa' into art , code logic is incorrect.</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Set key bug #21

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[Copilot]

Pull request overview

This PR fixes two critical bugs in the set method: a buffer overflow writing beyond allocated memory and a memory leak from incorrect delete operator usage. The fixes ensure correct memory allocation/deallocation patterns and add test coverage documenting C-string semantics behavior.

• Fixed buffer overflow by copying exactly key_len bytes instead of key_len + 1 into a key_len-sized buffer (line 126)
• Fixed memory leak by using delete[] instead of delete for array deallocation (line 200)
• Added test case documenting that "aa" and "aa\0" are identical keys due to C-string semantics

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
include/art/art.hpp	Fixes buffer overflow in root node creation (line 126) and memory leak in prefix reallocation (line 200)
test/art.cpp	Adds test case documenting C-string semantics for keys with explicit trailing nulls