resolveDownloadDirectory function

@visibleForTesting String resolveDownloadDirectory(BaseDirectory baseDirectory, String splitDirectory, String targetPath, { Context? context, })

Directory to hand background_downloader so the download lands exactly at targetPath.

Task.split maps an absolute path onto background_downloader's BaseDirectory model, but a target NOT under one of its recognized bases falls back to BaseDirectory.root with the drive/root STRIPPED from splitDirectory. The case that bites us is Windows' %LOCALAPPDATA%\flutter_gemma: LocalAppData is not a background_downloader base (path_provider maps applicationSupport → Roaming, applicationDocuments → Documents), so split returns root + Users\..\AppData\Local\flutter_gemma. On Windows the root base resolves to '' (not the drive), so the reconstructed filePath is $CWD-relative and the file lands in the wrong place while getReadTargetPath / validateModelFiles look at the absolute path — install() "succeeds" but isModelInstalled() stays false. Return the ABSOLUTE directory for the root case so the file lands at targetPath.

Non-root (recognized-base) targets keep splitDirectory verbatim — that value is stored on the DownloadTask, and the reclaim path (mobile_model_manager) RE-derives a task id from DownloadTask.directory via computeTaskId; keeping it the relative, container-UUID-independent split value is what keeps that recompute matching. (The task's own stored taskId is derived from the split triple regardless of this function.) The root case's absolute directory would make that recompute diverge, but it never reaches it: the reclaim is Android-only and Android always resolves to a recognized base, so it never takes the root fallback.

context is injected only by tests so Windows path semantics can be exercised on a POSIX CI host; production uses the host p.context (Windows on the affected platform), so dirname uses the correct separators.

Implementation

@visibleForTesting
String resolveDownloadDirectory(
  BaseDirectory baseDirectory,
  String splitDirectory,
  String targetPath, {
  p.Context? context,
}) => baseDirectory == BaseDirectory.root
    ? (context ?? p.context).dirname(targetPath)
    : splitDirectory;