Enable ConnectionPoolV2 by default

[mdaigle]

Description

Changes the default connection pool from WaitHandleDbConnectionPool to ChannelDbConnectionPool. Applications can restore the legacy pool explicitly:

AppContext.SetSwitch("Switch.Microsoft.Data.SqlClient.UseConnectionPoolV2", false);

The full CI matrix exposed and this PR retains fixes for:

• Reacquiring normally when connection resiliency has already removed the broken connection's pool slot.
• Honoring an expired caller timeout before physical connection creation.
• Unwrapping and observing Task<Task> in TvpTest.TestPacketNumberWraparound.
• Replacing timing-based ambient transaction coverage with direct sync and async Enlist=false public-API coverage.

Flaky-test stabilization and quarantine changes discovered during investigation were removed for a separate PR. No public APIs change.

A best-of-three comparison covered 162 benchmarks with a 10% threshold. It found 41 improvements, 9 confirmed regressions, and 3 unconfirmed regressions. The strongest remaining regression signals are fixed overhead in pooled OpenAsync, synchronization in rapid open/close loops, and contention when MaxPoolSize is small relative to concurrency.

Issues

Supersedes #4537, which used a fork-backed branch.

The prerequisite emancipated connection reclamation fix merged through #4529.

Testing

• ChannelDbConnectionPoolTest: 64/64 passed on net9.0 after rebasing onto main.
• ChannelDbConnectionPoolReplaceConnectionTest: 16/16 passed on net8.0 and net9.0.
• Open_WithEnlistDisabled_DoesNotEnlistInAmbientTransaction: sync and async cases passed on net8.0 and net9.0.
• Default-switch and connection-pool unit selection: 332/332 passed on net8.0, net9.0, and net10.0.
• The seven CI legs affected by the TVP test passed after the test fix.
• A prior complete matrix had only unrelated macOS SQL Server container, ARM64 Azure SQL TLS, and managed identity infrastructure failures. The replacement PR starts a fresh current-head matrix.

Guidelines

Please review the contribution guidelines before submitting a pull request:

• Contributing
• Code of Conduct
• Best Practices
• Coding Style
• Review Process

[Copilot]

Pull request overview

Enables ChannelDbConnectionPool (ConnectionPoolV2) as the default connection pooling implementation via LocalAppContextSwitches.UseConnectionPoolV2, updating documentation and tests to reflect the new default and to cover key behavioral/regression scenarios discovered during prior CI/perf validation.

Changes:

• Flip UseConnectionPoolV2 default to true and update switch documentation + default-value unit test.
• Add/adjust targeted tests covering (a) Enlist=false behavior under an ambient transaction (sync + async), (b) “expired timeout prevents physical creation”, and (c) resiliency replacement when the old connection was already removed.
• Fix TvpTest.TestPacketNumberWraparound to correctly unwrap/observe the nested Task<Task> produced by Task.Factory.StartNew.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/LocalAppContextSwitches.cs	Sets UseConnectionPoolV2 default to true and updates its XML doc comment accordingly.
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs	Adjusts ReplaceConnection to reacquire via the normal path when the old connection is no longer in this pool.
src/Microsoft.Data.SqlClient/tests/UnitTests/Microsoft/Data/SqlClient/LocalAppContextSwitchesTest.cs	Updates the default-switch assertion to expect UseConnectionPoolV2 == true.
src/Microsoft.Data.SqlClient/tests/UnitTests/ConnectionPool/ChannelDbConnectionPoolTest.cs	Adds deterministic coverage ensuring expired timeouts prevent physical connection creation attempts.
src/Microsoft.Data.SqlClient/tests/UnitTests/ConnectionPool/ChannelDbConnectionPoolReplaceConnectionTest.cs	Adds coverage for replacement after the old connection has already been removed; adds a test helper method to doom stub connections.
src/Microsoft.Data.SqlClient/tests/UnitTests/SimulatedServerTests/ConnectionTests.cs	Adds an in-proc TDS server test verifying Enlist=false prevents ambient transaction enlistment for both Open and OpenAsync.
src/Microsoft.Data.SqlClient/tests/ManualTests/SQL/ParameterTest/TvpTest.cs	Fixes nested-task handling from Task.Factory.StartNew by unwrapping and explicitly observing failures.
src/Microsoft.Data.SqlClient/tests/FunctionalTests/AmbientTransactionFailureTest.cs	Removes the prior timing/ambient-transaction-based functional test (replaced by direct public-API coverage elsewhere in this PR).
.github/instructions/features.instructions.md	Updates the switch table to reflect UseConnectionPoolV2 defaulting to true and how to restore V1.

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