🐛 Fix flaky Windows dialog thread test

[vinceglb]

Summary

• Test the Windows dialog worker base name and daemon flag at thread creation time.
• Verify executor work runs on the factory-created thread without observing a coroutine-decorated live name.
• Keep the dedicated single-thread STA executor, COM balancing, serialization, and real Windows dialog integration coverage unchanged.

Root cause

Coroutine debug instrumentation temporarily decorates the active thread name with a coroutine identifier. The previous test returned the mutable thread from withContext and could read its name before that decoration was restored.

Related to #600.

Testing

• ./gradlew :filekit-dialogs:jvmTest --tests *WindowsDialogExecutorTest* --rerun-tasks -Dkotlinx.coroutines.debug=on --no-daemon
• ./gradlew :filekit-dialogs:jvmTest --tests *WindowsDialogExecutorTest* --rerun-tasks -Dkotlinx.coroutines.debug=off --no-daemon
• ./gradlew :filekit-dialogs:check --no-daemon
• ktlint on the changed Kotlin files