Allow flutter SDK binary to be a symlink (#9924)

* Fix SDK path resolution when Flutter on PATH is a symlink

When running `dt update-flutter-sdk --update-on-path`, `FlutterSdk.findFromPathEnvironmentVariable` failed if `flutter` on PATH was a symlink (e.g. `/usr/local/bin/flutter`). It stripped `bin/flutter` from the unresolved symlink path, resulting in `/usr/local` being treated as the SDK root directory.

This change:
- Updates `FlutterSdk.findFromPath` to resolve symbolic links before determining the SDK root directory.
- Updates `UpdateFlutterSdkCommand` to use the remote returned by `findRemote` instead of hardcoding 'upstream'.
- Adds unit tests in `tool/test/model_test.dart` for `FlutterSdk.findFromPath`.

* Revert change to "findRemote" behavior

* Revert change to "findRemote" behavior

* Simplify symlink resolution using try-catch fallback in FlutterSdk.findFromPath

* Add release note for dt sync symlink fix

* format

* Revert changes to release notes

* extract repeated calls to path.dirname

* Avoid explicit variable type declaration.

* Remove ignore for DCM