Fix experimental MessagePack round trips with current msgpack

[Shubham-Padkonde]

MessagePack round trips currently raise TypeError with msgpack 1.x because the encoder and decoder still pass the removed encoding argument. This updates the experimental serializer to use binary-aware encoding and decoding, while preserving arbitrary bytes written using the legacy raw representation.

The decoder now also forwards alias_validators and strict by name: positional forwarding was binding them to the wrong arguments after caller_permissions was added. MessagePack is included in the test dependencies so this path is exercised.

Validation: all 198 tests pass on Linux with msgpack 1.2.3. Nine new regression cases failed before the fix and pass afterward, covering Unicode, binary collections, legacy raw bytes, alias validation, and strict decoding. Flake8 passes for the changed Python files. Two existing tests encounter Windows-specific filesystem/whitespace failures; the complete Linux run passes.

[ineil-dbx]

Hi @Shubham-Padkonde, I’ve raised this with our engineering team for further review. They’ll be able to review the pull request and address it as appropriate.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.36%. Comparing base (1ad079b) to head (6d27c6a).
⚠️ Report is 36 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #396      +/-   ##
==========================================
+ Coverage   49.98%   52.36%   +2.37%     
==========================================
  Files          37       42       +5     
  Lines        8808    11451    +2643     
  Branches     1892     2393     +501     
==========================================
+ Hits         4403     5996    +1593     
- Misses       4087     4922     +835     
- Partials      318      533     +215     

Flag	Coverage Δ
unit	52.36% <100.00%> (+2.37%)	⬆️

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