[6.1] Fix connection pool concurrency issue (#3632)

[cheenamalhotra]

Ports #3632 to release/6.1

[Copilot]

Pull Request Overview

This PR addresses a connection pool concurrency issue by improving semaphore handling and includes comprehensive stress tests to validate the fix. It ports the changes from PR #3632 to the release/6.1 branch.

Key Changes

• Refactored semaphore acquisition and release pattern to use RAII-style resource management
• Added comprehensive connection pool stress tests with both synchronous and asynchronous scenarios
• Simplified error handling logic in pool creation requests

Reviewed Changes

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

File	Description
WaitHandleDbConnectionPool.cs	Introduces SemaphoreHolder class for safe semaphore management and refactors pool creation logic
ConnectionPoolStressTest.cs	New comprehensive test class for validating connection pool behavior under concurrent load
Microsoft.Data.SqlClient.ManualTesting.Tests.csproj	Adds the new stress test to the project compilation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.06%. Comparing base (7a7b54e) to head (1988750).
⚠️ Report is 14 commits behind head on release/6.1.

Additional details and impacted files

@@               Coverage Diff               @@
##           release/6.1    #3653      +/-   ##
===============================================
+ Coverage        69.69%   70.06%   +0.37%     
===============================================
  Files              281      279       -2     
  Lines            62413    61746     -667     
===============================================
- Hits             43500    43264     -236     
+ Misses           18913    18482     -431     

Flag	Coverage Δ
addons	90.82% <ø> (ø)
netcore	73.08% <100.00%> (+0.32%)	⬆️
netfx	69.81% <100.00%> (+0.56%)	⬆️

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