Accept project root paths in debug sessions for sidebar

[DanTup]

See #6709

Dart-Code will pass a project root* for debug sessions:

* if it's able to compute one - which it always should for the cases we care about, but there are some edge cases that mean the field might be missing.

[kenzieschmoll]

just to be clear, this is the path that should contain the .dart_tool/ folder?

[DanTup]

It's the folder where we found the nearest pubspec.yaml while walking upwards. I think that's always the same thing?

I've updated the comment to mention this explicitly ("The full path to the root of this project (the folder that contains the pubspec.yaml).")

[auto-submit]

auto label is removed for flutter/devtools/6715, due to - The status or check suite devtools_app integration-test integration_dart2js - flutter - shard 1/3 has failed. Please fix the issues identified (or deflake) before re-applying this label.

[DanTup]

Failure looks like it might be a repeat of #6289 which was thought to be fixed by #6291:

FlutterDriveProcess - Failure Details:
FlutterDriveProcess - Failure in method: Debugger panel
FlutterDriveProcess - ══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞═════════════════
FlutterDriveProcess - The following TestFailure was thrown running a test:
FlutterDriveProcess - Expected: exactly one matching candidate
Unhandled exception:
FlutterDriveProcess -   Actual: _DescendantWidgetFinder:<Found 0 widgets with widget
Exception: Test 'Debugger panel' failed: ══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞═════════════════
FlutterDriveProcess - matching predicate descending from widgets with type "CallStack":
The following TestFailure was thrown running a test:
FlutterDriveProcess - []>
Expected: exactly one matching candidate
FlutterDriveProcess -    Which: means none were found but one was expected
  Actual: _DescendantWidgetFinder:<Found 0 widgets with widget
FlutterDriveProcess - 
matching predicate descending from widgets with type "CallStack":
FlutterDriveProcess - When the exception was thrown, this was the stack:
[]>
FlutterDriveProcess - dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 297:3  throw_
   Which: means none were found but one was expected
FlutterDriveProcess - packages/matcher/src/expect/prints_matcher.dart.js 436:22                    fail

FlutterDriveProcess - packages/matcher/src/expect/prints_matcher.dart.js 433:12                    _expect
When the exception was thrown, this was the stack:
FlutterDriveProcess - packages/matcher/src/expect/prints_matcher.dart.js 368:12                    expect$
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 297:3  throw_

https://github.com/flutter/devtools/actions/runs/6861908888/job/18658577906?pr=6715

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