Port #3929 to release/6.1

[apoorvdeshmukh]

Description

Ports #3929 to release/6.1

Issues

Fixes #3941

Testing

Issue impacted only the netcore app and not the netfx app.
Added unit test to validate the code changes.
E2E Validation using the repro provided in #3523 and env setup as mentioned in #3929

Without the fix

Data Source=tcp:XXXXXX,XXXX;Initial Catalog=Demo2;User ID=XX;Password=XXXXXXXX;Connect Timeout=10;Encrypt=True;Trust Server Certificate=True;Application Name="Connection test"
Connection successful!
Time taken to connect: 5185 ms ms

With the fix

Data Source=tcp:XXXXXX,XXXX;Initial Catalog=Demo2;User ID=XX;Password=XXXXXXXX;Connect Timeout=10;Encrypt=True;Trust Server Certificate=True;Application Name="Connection test"
Connection successful!
Time taken to connect: 263 ms

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[Copilot]

Pull request overview

Backports PR #3929 to release/6.1, adjusting native SNI connection setup so SPN generation is only triggered for integrated security, avoiding unnecessary DNS/SPN work for non-integrated auth modes (e.g., SQL auth) on Windows native SNI.

Changes:

• Normalize serverSPN based on authentication mode before calling into native SNI (empty string only used to trigger SPN generation for integrated security; otherwise avoid generation).
• Adjust resolvedSpn creation to only occur when a non-empty SPN is present after native SNI setup.
• Add a unit test intended to validate SPN normalization behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/TdsParserStateObjectNative.cs	Introduces SPN normalization helper and updates native handle creation logic to avoid unintended SPN generation for non-integrated auth.
src/Microsoft.Data.SqlClient/tests/UnitTests/Microsoft/Data/SqlClient/TdsParserStateObjectNativeTests.cs	Adds a unit test for SPN normalization logic (currently gated in a way that likely prevents it from compiling/running).