Fix partial file download loss on multi-chunk resume

When a large file spans multiple HTTP requests, continuation chunks
arrive with is_first=false. But the StreamingContext resets between
requests, leaving file_handle=null, so the chunk handler silently
drops the remaining data. The importer then reports success with a
truncated file.

Fix: if a partially downloaded file is tracked in state, re-open it
in append mode before streaming begins so continuation chunks are
written correctly.

Also give the large-single-file E2E test its own dedicated site to
prevent interference from test-28's concurrent-error hooks which
modify files on the shared import-failures site.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: adamziel

importer/import.php

@@ -2768,6 +2768,26 @@ private function download_file_fetch(
         $context->file_path = null;
         $context->file_ctime = null;
 
+        // Resume recovery: if a file was partially downloaded in a previous
+        // request, re-open it in append mode so continuation chunks (where
+        // is_first=false) can still be written.  Without this, the context
+        // starts with file_handle=null and non-first chunks are silently dropped.
+        if ($tracked_file !== null && $tracked_bytes !== null && file_exists($tracked_file)) {
+            $context->file_handle = fopen($tracked_file, "ab");
+            if ($context->file_handle) {
+                $context->file_path = $tracked_file;
+                $context->file_bytes_written = $tracked_bytes;
+                $this->audit_log(
+                    sprintf(
+                        "RESUME FILE | Re-opened %s at %d bytes for continued download",
+                        $tracked_file,
+                        $tracked_bytes,
+                    ),
+                    true,
+                );
+            }
+        }
+
         $context->on_chunk = function ($chunk) use (
             &$cursor,
             &$complete,

tests/e2e/site-registry.json

@@ -26,6 +26,7 @@
     "error-chunks":      { "port": 8099 },
     "import-failures":   { "port": 8100 },
     "follow-symlinks":   { "port": 8101 },
-    "sqlite":            { "port": 8102 }
+    "sqlite":            { "port": 8102 },
+    "large-single-file": { "port": 8103 }
   }
 }

tests/e2e/tests/import-24-large-single-file.test.js

@@ -17,7 +17,7 @@ import {
 import { ensureSite } from '../lib/site-setup.js';
 
 describe('Import: Large Single File', { timeout: 180000 }, () => {
-    const site = 'import-failures';
+    const site = 'large-single-file';
     let tempDir;
     const largeName = 'test-data/large-12mb.bin';