Ensure correct SPN when calling SspiContextProvider

[twsouthwick]

This change does the following:

• Only initialize when handshake is complete
• The serverSpn field is now local to the method
• Adds some bookkeeping to ensure we're only using the SPN that has previously generated a context once one has been created.
• Reworks the SNI pathways on .NET Core to only return the SPN that is needed. On .NET Core, it was returning an array but only using a single value from that. This refactors that to be more readable to just return the value used.

See comment: See #2454 (comment)

Part of #2253

[codecov]

Codecov Report

Attention: Patch coverage is 77.35849% with 12 lines in your changes missing coverage. Please review.

Project coverage is 58.79%. Comparing base (832e8da) to head (3ca3de6).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
...crosoft/Data/SqlClient/SSPI/SspiContextProvider.cs	73.68%	5 Missing ⚠️
...ata/SqlClient/SSPI/NegotiateSspiContextProvider.cs	70.00%	3 Missing ⚠️
...nt/netfx/src/Microsoft/Data/SqlClient/TdsParser.cs	66.66%	2 Missing ⚠️
...tcore/src/Microsoft/Data/SqlClient/SNI/SNIProxy.cs	80.00%	1 Missing ⚠️
.../netcore/src/Microsoft/Data/SqlClient/TdsParser.cs	80.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (832e8da) and HEAD (3ca3de6). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (832e8da)	HEAD (3ca3de6)
addons	1	0

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3347      +/-   ##
==========================================
- Coverage   65.12%   58.79%   -6.34%     
==========================================
  Files         300      293       -7     
  Lines       65379    65213     -166     
==========================================
- Hits        42578    38340    -4238     
- Misses      22801    26873    +4072     

Flag	Coverage Δ
addons	?
netcore	62.93% <78.72%> (-5.41%)	⬇️
netfx	59.73% <75.86%> (-6.65%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[paulmedynski]

Some comments about handling multipe and single SPNs.

[paulmedynski]

Whoops - hit Comment instead of Request Changes

[paulmedynski]

Looks like some legitimate test failures for net8.0 and net9.0:

     System.InvalidCastException : Unable to cast object of type 'Microsoft.Data.SqlClient.SNI.ResolvedServerSpn' to type 'System.String'.
    Stack Trace:
       at Microsoft.Data.SqlClient.ManualTesting.Tests.InstanceNameTest.GetSPNInfo(String dataSource, String inInstanceName) in /_/src/Microsoft.Data.SqlClient/tests/ManualTests/SQL/InstanceNameTest/InstanceNameTest.cs:line 215