fix: compute actual string length in set_owning heap path

[a2Fsa2k]

When set_owning(str, len) is called with a len larger than the actual null-terminated length of str, and the allocation falls on the heap path (len > short_length_limit), h_.size_ was set to the full len rather than the actual string length. This causes size() to report incorrect values.

Example: set_owning("", 32) would report size() == 32 instead of size() == 0 on the heap path, inconsistent with the short string path which correctly returns 0.

The fix computes the actual string length via std::char_traits<CharT>::length(str) and uses min(actual_length, len) for h_.size_.

Fixes #190.

[a2Fsa2k]

Note: the CI build failures (in vector_test.cc and nvec_test.cc) are pre-existing on the master branch since the add free_self_allocated commit on April 5, 2026. They are unrelated to this change — my fix only modifies set_owning() in string.h.

@felixguendling whenever you get a chance to review, the fix addresses the inconsistent size() reported by set_owning() when the allocated capacity exceeds the actual string length on the heap path (issue #190).