Fix serialization of native integral types

[swdaa]

Summary

Fix serialization of native integral types that are not aliases of the
fixed-width integer types handled by the existing overloads.

On macOS, std::size_t is unsigned long, while std::uint64_t may be
a different underlying type. As a result, std::size_t could fall through
to the custom-object overload and attempt to call value.pack() /
value.unpack().

This change routes otherwise-unhandled integral types through the existing
fixed-width integer serialization overloads and excludes integral types from
the custom-object fallback.

Testing

• Built the project with Clang on macOS
• Existing std::vector<std::size_t> array test now compiles
• All tests pass with ctest --test-dir build --output-on-failure

[Copilot]

Pull request overview

Fixes MsgPack serialization for native integral types (e.g., std::size_t) whose underlying type may not be one of the fixed-width integer typedefs, preventing them from incorrectly falling into the custom-object pack()/unpack() fallback.

Changes:

• Added pack_type / unpack_type overloads to route otherwise-unhandled std::integral types through existing fixed-width integer serialization.
• Excluded integral types from the custom-object fallback overloads so they don’t attempt value.pack() / value.unpack().

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.91%. Comparing base (53f2594) to head (ab48c59).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #11   +/-   ##
=======================================
  Coverage   93.91%   93.91%           
=======================================
  Files           1        1           
  Lines         493      493           
=======================================
  Hits          463      463           
  Misses         30       30           

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