Fix #407

[goweii]

Fix multi app DTD test on Windows by ensuring orphaned test apps and DTD connections shut down cleanly.

This prevents temp directory cleanup failures when the multi-connection DTD tests run on Windows.

• Fixes Fix multi app DTD test on windows #407
• Ensures dtd_multiple_connection_test.dart teardown completes properly on Windows
• Stabilizes multiple DTD connection test behavior across platforms

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Code Review

This pull request updates the test harness to ensure processes are properly terminated on Windows by polling for the PID, and re-enables a previously skipped test. The review feedback identifies a potential tight loop in the termination logic that could cause high CPU usage and suggests optimizing the PID check by using command-line filters instead of parsing the entire process list.

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