fix: avoid double-init of late final _root in NodeDriveService (1.27.4)

[gmpassos]

Problem

Starting a Windows node and opening a drive session crashed with:

LateInitializationError: Field '_root@...' has already been initialized.
#1  NodeDriveService._root= (node_drive_service.dart)
#2  NodeDriveService.run (node_drive_service.dart:78:7)

This is a regression from #72 / commit bcb029b ("translate MSYS drive-mount root on Windows nodes", v1.27.3). _root is declared late final String (one assignment only), but the new MSYS-translation code assigned it twice on Windows when the path starts with /:

_root = expandUserHome(request.command ?? '.');   // first assignment
if (Platform.isWindows && _root.startsWith('/')) {
  _root = windowsPathFromMsys(_root);              // second assignment -> crash
}

The crash fires on exactly the Windows + MSYS-root case the 1.27.3 fix was meant to support: the session opens, then the drive service dies, taking the node down.

Fix

Resolve the root into a single value via a pure, platform-parameterized static helper NodeDriveService.resolveRoot(command, isWindows: ...), assigned to _root exactly once. Extracting the helper also makes the Windows-only branch unit-testable off Windows.

Tests

• New test/unit/node/node_drive_service_test.dart covering MSYS translation on Windows, POSIX pass-through (regression guard), already-Windows / non-drive paths, null default, and idempotency.
• All new tests + version_test pass; dart format + dart analyze clean.

Version

Bumped 1.27.3 -> 1.27.4 (pubspec.yaml, lib/src/version.dart) + CHANGELOG entry.

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