Stabilize Postgres reconnect test against reconnect timing race

[glen-84]

Summary

• Reconnect_Should_ReconnectWhenConnectionIsClosed waited a fixed 1 second (SpinWait.SpinUntil) for the background reconnect to swap in a new connection, then asserted unconditionally. Under CI load the reconnect can exceed that window, producing an intermittent Assert.NotEqual failure.
• Replaced the fixed-timeout wait with a deterministic await on a TaskCompletionSource signalled by the reconnect's onConnect callback (10s ceiling), mirroring the sibling Reconnect_Should_CallOnDisconnect_When_ConnectionIsClosed test. Connection is assigned before onConnect is awaited, so the new connection is guaranteed to be in place when the assertion runs.

Test plan

• Built HotChocolate.Subscriptions.Postgres.Tests and ran the affected test 10× locally on net10.0: 10/10 passed.
• The wait now keys off the actual reconnect event rather than a fixed timeout, removing the timing race on every target framework.

[Copilot]

Pull request overview

This PR stabilizes the ResilientNpgsqlConnection Postgres reconnect test by removing a fixed-duration wait that can flake under CI load, and replacing it with a deterministic await tied to the actual reconnect event.

Changes:

• Replaced SpinWait.SpinUntil(..., 1s) with a TaskCompletionSource that completes when the second onConnect fires (reconnect), with a 10s ceiling.
• Ensured the assertion runs only after the reconnect callback indicates the connection swap has occurred, matching the pattern used by the sibling reconnect test.

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[github-code-quality]

Code Coverage Overview

Languages: C#

C# / code-coverage/dotnet

The overall coverage in the gai/stabilize-postgr... branch remains at 49%, unchanged from the main branch.

Show a code coverage summary of the most impacted files.

File	main 9e5f4b0	gai/stabilize-postgr... 7b9947c	+/-
/home/runner/wo...gerMessage.g.cs	19%	15%	-4%
/home/runner/wo...gerMessage.g.cs	28%	25%	-3%
/home/runner/wo...eiveEndpoint.cs	80%	77%	-3%
/home/runner/wo...geDispatcher.cs	77%	75%	-2%
/home/runner/wo...xecutionNode.cs	87%	85%	-2%
/home/runner/wo...tocolHandler.cs	85%	85%	0%
/home/runner/wo.../ThrowHelper.cs	26%	26%	0%
/home/runner/wo...text.Pooling.cs	99%	100%	+1%
/home/runner/wo...cutorManager.cs	94%	95%	+1%
/home/runner/wo...qlConnection.cs	89%	92%	+3%

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