Migrate to slnx solution file format

[paulmedynski]

Description

I used Opus 4.6 to plan a migration from the legacy sln format to the modern slnx format, and it didn't find any legitimate issues. I encourage folks to try out the new slnx file on this branch with their IDEs and tooling.

Benefits include:

• slnx is the future, so we should embrace it.
• Much simpler diffs in PRs.
• No more GUID overload.

Testing

Our pipelines don't use the solution file, but they can check for any other regressions.

[Copilot]

Pull request overview

This PR migrates the repository’s primary developer solution from the legacy Visual Studio .sln format to the newer XML-based .slnx format under src/, aiming to simplify solution diffs and reduce GUID-heavy churn.

Changes:

• Added src/Microsoft.Data.SqlClient.slnx as the solution definition in .slnx format.
• Removed the legacy src/Microsoft.Data.SqlClient.sln.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Microsoft.Data.SqlClient.slnx	New XML-based solution file containing projects, folders, and solution items.
src/Microsoft.Data.SqlClient.sln	Deleted the legacy .sln solution previously used by IDEs/tooling.

[benrr101]

This is probably a good change :) I'm tired of the guid changes everywhere. Though I hope we can come up with a better folder scheme soonish.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.08%. Comparing base (07a9280) to head (81761f7).
⚠️ Report is 2 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (07a9280) and HEAD (81761f7). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (07a9280)	HEAD (81761f7)
CI-SqlClient	2	0

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4070      +/-   ##
==========================================
- Coverage   75.17%   68.08%   -7.10%     
==========================================
  Files         280      275       -5     
  Lines       43830    66924   +23094     
==========================================
+ Hits        32951    45563   +12612     
- Misses      10879    21361   +10482     

Flag	Coverage Δ
CI-SqlClient	?
PR-SqlClient-Project	68.08% <ø> (?)

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