DevTools extensions not discovered when app entrypoint is under custom directory

### Sumary

DevTools extension discovery fails if the main isolate root library is under a custom test directory (`patrol_test/` in our case) that is not recognized by DevTools’ package root heuristic. Extensions appear when tests are moved to `integration_test/` which is a recognized root.

### Context
I'm working on [Patrol](https://github.com/leancodepl/patrol) which has it's own devtools extension. We recently changed the default directory where all the tests are stored from `integration_test` to `patrol_test`. After doing so, we then found out that our devtools extension is not found when running tests.

### Notes
DevTools root detection uses packageRootFromFileUriString, which only recognizes lib, bin, integration_test, test, benchmark, example as valid top‑level dirs. Custom test dirs like patrol_test/ are not recognized, so the root resolves incorrectly and package_config.json lookup fails.

Would it be possible to make it more configurable so that we could point to `patrol_test` directory in our case?

Start with DevTools' packageRootFromFileUriString root-detection logic and trace how it finds package_config.json for an entrypoint under patrol_test/. Compare that behavior with recognized directories such as integration_test/ and test/. Done means DevTools can discover extensions when the app entrypoint is under a custom test directory, while preserving existing root detection.

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