Guard StringUtil::StartsWith against prefix longer than string

[Chessing234]

Bug

`StringUtil::StartsWith` in `src/common/util/string_util.cpp` uses the 3-argument form of `std::equal`:

```cpp
auto StringUtil::StartsWith(const std::string &str, const std::string &prefix) -> bool {
return std::equal(prefix.begin(), prefix.end(), str.begin());
}
```

When `prefix.size() > str.size()`, this reads `prefix.size()` characters starting at `str.begin()`, walking past the end of `str`. That's undefined behaviour.

Root cause

No length check. The sibling `EndsWith` just below it already has the guard:

```cpp
auto StringUtil::EndsWith(const std::string &str, const std::string &suffix) -> bool {
if (suffix.size() > str.size()) {
return false;
}
return std::equal(suffix.rbegin(), suffix.rend(), str.rbegin());
}
```

`StartsWith` was written without it.

The bad path is trivially reachable from the interactive shells. In `tools/shell/shell.cpp` and `tools/nc-shell/nc-shell.cpp` the read loop ends when:

```cpp
if (bustub::StringUtil::EndsWith(query, ";") || bustub::StringUtil::StartsWith(query, "\\")) {
break;
}
```

and `query` can still be empty (e.g. the user presses Enter on a fresh prompt), which means `StartsWith("", "\\")` reads one byte past the start of an empty string. Other call sites (`bustub_instance.cpp` checking `\\dbgmvcc`/`\\txn`, `plan_table_ref.cpp` checking `__`/`__mock` table-name prefixes, `bind_insert.cpp`, `sqllogictest/sqllogictest.cpp` option parsing) all pass user-controlled strings that can be shorter than the prefix.

Fix

Add the same size guard `EndsWith` already uses:

```cpp
if (prefix.size() > str.size()) {
return false;
}
```

3 lines added, matches the `EndsWith` pattern a few lines below, keeps behaviour identical whenever `str` is at least as long as `prefix`.