Fix SmtpClientTest zero timeout hang

[rzikm]

This reverts part of the SmtpTransport changes from #115722 which caused timeouts and test suite crashes of System.Net.Mail.Tests.SmtpClientTest.TestZeroTimeout test.

Zero timeout values are weird, but for now the easier step is to preserve the old behavior than consider making breaking changes.

[Copilot]

Pull Request Overview

This PR restores the previous SmtpTransport abort behavior to avoid hangs when a zero timeout is specified, preserving compatibility for the existing TestZeroTimeout test.

• Introduces a _shouldAbort flag to defer aborting until after the connection is established
• Updates Abort() and GetConnectionAsync to use the deferred abort logic

Comments suppressed due to low confidence (1)

src/libraries/System.Net.Mail/src/System/Net/Mail/SmtpTransport.cs:22

• Consider adding a unit test to cover the scenario where Abort() is called before GetConnectionAsync() has created a connection to ensure the deferred abort logic behaves as expected and avoids regressions.

private bool _shouldAbort;

[Copilot]

Add an inline comment or XML doc summary for _shouldAbort to explain that this flag defers the abort until after the connection is established, clarifying its purpose for future maintainers.

Suggested change

	private ICredentialsByHost? _credentials;
	private ICredentialsByHost? _credentials;
	// Indicates whether the operation should be aborted. This flag defers the abort until after the connection is established.

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