[Fix] Use null-terminated string for symbol lookup in composite dependency analysis

[Fix] Use null-terminated string for symbol lookup in composite dependency analysis

In composite_dep_callback, atom->begin from rspamd_ftok_t is not null-terminated,
but was being passed directly to symbol_needs_second_pass() which calls
rspamd_symcache_get_symbol_flags() expecting a null-terminated C string.

This could cause incorrect symbol lookups or undefined behavior. Fix by creating
a std::string to ensure null-termination before passing to the C API.