Fixed issue that closed stream when using StreamWriter in SyntaxPrinter.PrintToAsync

[glen-84]

Summary of the changes (Less than 80 chars)

• Leave stream open when using StreamWriter in SyntaxPrinter#PrintToAsync.

Closes #7631

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📓 Notes:

• When FileSystemOperationDocumentStorage#SaveInternalAsync is called with a document that is an instance of OperationDocument, WriteToAsync calls PrintToAsync, which uses a StreamWriter that disposes the stream. The stream is then accessed again in order to flush it, which throws.
• The stream should be disposed by the caller, so I've set leaveOpen to true.

❓ Questions:

• Is there a simpler way to create the OperationRequest in the test?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.77%. Comparing base (6c9d385) to head (914c439).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7657      +/-   ##
==========================================
+ Coverage   73.75%   73.77%   +0.02%     
==========================================
  Files        2922     2922              
  Lines      150896   150895       -1     
  Branches    17550    17549       -1     
==========================================
+ Hits       111294   111327      +33     
+ Misses      33781    33748      -33     
+ Partials     5821     5820       -1     

Flag	Coverage Δ
unittests	73.77% <ø> (+0.02%)	⬆️

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