Add testMode config to debug tests via the DAP --test flag

[FrantisekGazo]

What

Adds an optional boolean testMode to the debug configuration. When true, the extension spawns the debug adapter with the --test flag ([fvm] flutter|dart debug_adapter --test), so program runs via flutter test / dart test instead of flutter run / dart run. Works for both "type": "flutter" and "type": "dart". Default false — existing configs are unaffected.

{
  "label": "Debug current test file",
  "adapter": "Dart",
  "type": "flutter",
  "request": "launch",
  "program": "$ZED_FILE",
  "cwd": "$ZED_DIRNAME",
  "testMode": true
}

Why

There was previously no way to debug a test the way it actually executes under flutter test (headless flutter_tester, AutomatedTestWidgetsFlutterBinding, goldens, …). The only workaround was flutter test --start-paused plus a manual attach with a freshly pasted vmServiceUri on every run. The Dart/Flutter SDKs ship this as a mode of the same adapter — see the Flutter DAP README (--[no-]test); the extension just never exposed it.

Changes

• src/dart.rs — read testMode; append --test to the adapter arguments; skip the deviceId → toolArgs -d <id> injection in test mode (flutter test runs headless — device selection is a flutter run concept, and users who need -d for integration tests can still pass it via toolArgs).
• debug_adapter_schemas/Dart.json — declare testMode (schema has additionalProperties: false); remove the dead "test" value from the flutterMode enum — the DAP treats flutterMode as a build mode (debug/profile/release), so "test" never did anything and would now be mistaken for this switch.
• README.md — "Debugging tests" section with the config above.

In test mode toolArgs keep their existing meaning — the DAP forwards them to flutter test / dart test (e.g. --plain-name, --tags).

Tested

• flutter + testMode: true on a widget test: headless flutter test run, breakpoints/stepping/variables work; toolArgs: ["--plain-name", …] narrows to a single test.
• Regression: existing flutter run launch and attach configs behave unchanged with testMode unset.