refactor: add string_array_to_vector helper

[milyin]

Link::get_interfaces() converts an owned C string array into a vector and releases the array manually. Extract that conversion into interop::detail::string_array_to_vector, which consumes the array and uses a scope guard to release it during exception unwinding as well.

Reuse the helper in Link::get_interfaces() and add regression coverage for empty arrays, empty strings, embedded NUL bytes, and independent C++ string ownership after the C array is released. Public API signatures are unchanged.

Validation:

• zenoh-c: details_zenohc and strict warnings_zenohc tests passed.
• zenoh-pico: the standalone helper regression cases passed against the installed library.
• C++17 compilation with exceptions disabled passed for the helper on both backends.
• clang-format 18.1.3 and git diff --check passed.

Related to #815, which also changes the cleanup in Link::get_interfaces() as part of a broader conversion-guard refactor.