Change TryReadString to use TryReadBytesWithContinue

[Wraith2]

Fixes #3421

Digging into the replication provided I found that if the data type of the string column were changed from ntext to nvarchar(max) everything worked as expected. The reason for this is that the two types are handled differently internally despite being value equivalent to the user. nvarchar() is a plp type and ntext is not. non-plp strings pass through the TryReadString function and it was tripping up when hitting the continue point because it had not been altered to deal with it.

TryReadString was using TryReadByteArray and in previous work I have written a version of this which encapsulates continue capable logic, TryReadByteArrayWithContinue. I moved this method down from TdsParser to TdsParserStateObject so it sits at the same level as the original method and then called it from TryReadString. This resolves the problem with minimal changes and no new code.

• The ability to use the existing api with minor rearrangements is pleasing and I think promising that we've got all the tools we need to handle continue.
• The code coverage for strings is alarming. I thought that we would have a lot better coverage than we seem to. Strings are a very common path and yet we've found at least 3 combinations that I would have expected to have coverage.
• This really need to go into Preview 2 to make sure anyone testing the preview has as many fixes as possible and that we pick up any new issues cleanly with preview2. @dotnet/sqlclientdevteam @David-Engel

/cc @ErikEJ @Kobuntu

[paulmedynski]

/azp run

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Azure Pipelines successfully started running 2 pipeline(s).

[cheenamalhotra]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[ErikEJ]

LGTM

[cheenamalhotra]

Small change to remove old cleanup query, rest LGTM!

[benrr101]

Low stakes comments on the test more than anything

[paulmedynski]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[cheenamalhotra]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[codecov]

Codecov Report

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

Project coverage is 62.70%. Comparing base (1058566) to head (a8862a9).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
...nt/netfx/src/Microsoft/Data/SqlClient/TdsParser.cs	0.00%	1 Missing ⚠️

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HEAD has 1 upload less than BASE

Flag	BASE (1058566)	HEAD (a8862a9)
addons	1	0

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3422      +/-   ##
==========================================
- Coverage   68.43%   62.70%   -5.73%     
==========================================
  Files         299      285      -14     
  Lines       65418    63321    -2097     
==========================================
- Hits        44766    39706    -5060     
- Misses      20652    23615    +2963     

Flag	Coverage Δ
addons	?
netcore	67.00% <100.00%> (-5.57%)	⬇️
netfx	61.87% <95.00%> (-5.30%)	⬇️

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