COR-173 - Test IMAP idle interruption

[dbezverkhnii]

Note

Low Risk
Test-only changes with no production MailCore behavior modified; CI impact is limited to Darwin test runs.

Overview
Adds Darwin-only XCTest coverage for IMAP IDLE teardown, ported from upstream’s test-imap-idle.cpp “cancel wakes IDLE” scenario (the disconnect race half is omitted because the Swift API lacks the synchronous C++ session).

A local FakeIdleIMAPServer (POSIX listening socket on 127.0.0.1) drives MCOIMAPSession through LOGIN/LIST/SELECT into IDLE and observes DONE/close. Tests run work off the main thread so MailCore’s main-queue operation lifecycle can proceed.

testInterruptIdleEndsRunningIdle asserts that interruptIdle() ends a running IDLE and the server sees DONE.

testCancelAllOperationsWakesRunningIdle asserts that cancelAllOperations() should wake a blocked IDLE; it is wrapped in XCTExpectFailure until upstream merges the fix where IMAPIdleOperation::cancel() calls interruptIdle(), with cleanup via interruptIdle() when the expectation is not met.

Package.swift registers IMAPIdleCancellationTests.swift in the MailCoreTests target.

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