Fix StopLogCatMonitor cancellation

[ConnorQi01]

Summary

Fix StopLogCatMonitor so cancelling the device-selection QuickPick exits cleanly instead of leaving the command pending forever.

Proposed Changes

• Return early when the StopLogCatMonitor QuickPick selection is cancelled.
• Preserve existing behavior for no active monitors, one active monitor, and selected monitors.
• Add unit coverage for no monitors, single monitor, selected monitor, and cancelled selection paths.

Test Plan

pm run build

pm test (timed out after launching the VS Code test host with Error mutex already exists / EPIPE; the generated est/ExtensionTests.xml shows stopLogCatMonitorCommand ran 4 tests with 0 failures. Existing failures in that run were
n-extension commandsRegistered, packager timeout, and ipNotificationService timeout.)

Closes #2760

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