glz::get support for generic_i64 and generic_u64

[stephenberry]

Summary

Fixes JSON pointer lookups for glz::generic_i64 and glz::generic_u64 when using glz::get<> and glz::get_if<> (issue #2315).

Changes from main

• include/glaze/json/generic.hpp
  • Generalized seek_op specialization from glz::generic to glz::generic_json<Mode>.
  • Generalized navigate_to overloads to support all generic number modes.
  • Generalized container-deserializing glz::get overloads to support all generic_json<Mode> types.
• tests/json_test/generic_int_test.cpp
  • Added regression tests for JSON pointer access on both generic_i64 and generic_u64:
    • glz::get<bool>(..., "/Example/enabled")
    • glz::get<std::string>(..., "/Example/name")
    • glz::get_if<std::string>(..., "/Example/name")