fix: address review comments on core library hardening

[Copilot]

Follow-up to #1302 addressing five specific issues raised in code review of the core C/C++ hardening PR.

Changes

encode.c / encode.h

• Replace assert(src) / assert(dst) with null checks returning 0 in omega_encode_hex2bin
• Update API doc: dst requires (src_length / 2) + 1 bytes (null-terminated, consistent with bin2hex)

search.cpp

• Replace assert(0 < pattern_length) with early return nullptr when pattern_length <= 0, consistent with other guards in the same function

edit.cpp

• omega_edit_undo_last_change: add if (!session_ptr) { return 0; } before accessing session_ptr->models_ — omega_session_changes_paused(nullptr) returns 0, so a null session would otherwise enter the loop and crash
• omega_edit_redo_last_undo: same null guard; break out of the redo loop immediately on update_() failure, leaving the failed change in changes_undone so the session isn't advanced further in an error state

filesystem.cpp

• In omega_util_available_filename, check results of omega_util_dirname, omega_util_basename, and omega_util_file_extension for nullptr before constructing std::string — these can return nullptr for overlong paths, which previously caused UB

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