Fix PipeReader position after an async deserialization

[AArnott]

Prior to this fix, the MessagePackSerializer.DeserializeAsync<T>(PipeReader, ...) methods would leave the PipeReader in an undefined state.

Copilot reviewed 4 out of 6 changed files in this pull request and generated no comments.

Files not reviewed (2)

• src/Nerdbank.MessagePack/net8.0/PublicAPI.Unshipped.txt: Language not supported
• src/Nerdbank.MessagePack/netstandard2.0/PublicAPI.Unshipped.txt: Language not supported

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.34%. Comparing base (7c0b00e) to head (997b760).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Nerdbank.MessagePack/MessagePackAsyncReader.cs	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #240      +/-   ##
==========================================
+ Coverage   84.32%   84.34%   +0.01%     
==========================================
  Files         164      164              
  Lines       12889    12907      +18     
  Branches     1403     1405       +2     
==========================================
+ Hits        10869    10886      +17     
- Misses       1991     1992       +1     
  Partials       29       29              

Flag	Coverage Δ
Linux	82.56% <95.23%> (-0.01%)	⬇️
Windows	84.30% <95.23%> (+0.01%)	⬆️

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