Fix auto select main isolate

[rishika0212]

Fixes #9747

When DevTools connects to a test run, three isolates are present: the test runner (main), the test suite isolate where user code actually runs (test_suite:...), and vm-service. Previously, DevTools fell through to selecting the first isolate (main) as a fallback, so CPU profiling and other tools showed no data and the user's test code was running in a completely different isolate.

This changes _computeMainIsolate in IsolateManager to prefer thetest_suite: isolate when no Flutter extension or :main( isolate is found. The test_suite: prefix is set as a debugName by package:test_core when it spawns each test suite isolate, making it a reliable signal to detect test runs.

Before: CPU profiler showed empty results when connected to a test run.
After: CPU profiler correctly captures samples from the test suite isolate.

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