Flaky tests / lighter test queries

[mdaigle]

Marks a few flaky tests. Rewrites a manual test as a unit test. Reduces CPU/scan impact on a few high volume tests queries.

[Copilot]

Pull request overview

This pull request updates the SqlClient test suite by quarantining a small set of intermittently failing unit tests, replacing a manual transaction-pool cleanup scenario with a deterministic unit test, and reducing CPU/scan overhead in high-volume MARS manual tests (notably for Azure SQL environments).

Changes:

• Quarantines a few simulated-server transient fault/failover tests as Category=flaky with inline failure notes where helpful.
• Adds a deterministic unit test validating that pool pruning does not impact transacted connections, and exposes the pruning callback internally to support that test.
• Optimizes MARS manual test queries to avoid expensive system scanning (sys.databases, sp_who) while still producing sufficient observable request activity.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Microsoft.Data.SqlClient/tests/UnitTests/SimulatedServerTests/ConnectionTests.cs	Marks select transient-fault test cases as flaky to prevent intermittent CI failures.
src/Microsoft.Data.SqlClient/tests/UnitTests/SimulatedServerTests/ConnectionFailoverTests.cs	Quarantines an intermittently failing failover-related async test with a documented failure symptom.
src/Microsoft.Data.SqlClient/tests/UnitTests/ConnectionPool/WaitHandleDbConnectionPoolTransactionTest.cs	Adds a pruning-focused unit test asserting transacted connections are not pruned/destroyed.
src/Microsoft.Data.SqlClient/tests/ManualTests/SQL/MARSSessionPoolingTest/MarsSessionPoolingTest.cs	Replaces high-cost SQL text/SP usage with lighter alternatives to reduce CPU/scan impact while preserving test intent.
src/Microsoft.Data.SqlClient/tests/ManualTests/SQL/ConnectionPoolTest/TransactionPoolTest.cs	Removes the older manual transaction-cleanup test (scenario now covered deterministically via unit test).
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/WaitHandleDbConnectionPool.cs	Makes the pruning callback internal (with documentation) so unit tests can invoke pruning deterministically.