test: bootstrap the bundled Isar core for real-database tests

Isar.open only looks for libisar.so on the system loader path and in the
working directory, and neither holds it on a fresh checkout or in CI.
Resolve the binary shipped with isar_community_flutter_libs instead, so a
test can open a real Isar with no manual step and no network download.

Author: sneurlax

test/utilities/isar_test_core.dart

@@ -0,0 +1,60 @@
+import 'dart:convert';
+import 'dart:ffi';
+import 'dart:io';
+
+import 'package:isar_community/isar.dart';
+
+/// Point Isar at the core binary bundled in `isar_community_flutter_libs` so
+/// tests can open a real Isar instance anywhere, including CI.
+///
+/// Without this, `Isar.open` only looks for `libisar.so` on the system loader
+/// path or in the working directory; neither exists on a fresh checkout, and
+/// the alternative (`download: true`) would make the test suite hit the
+/// network. Call once from `setUpAll` before opening Isar. Repeat calls and
+/// calls from other test files are no-ops.
+Future<void> initializeIsarCoreForTests() async {
+  final String? relativeLibPath = switch (Abi.current()) {
+    Abi.linuxX64 || Abi.linuxArm64 => "linux/libisar.so",
+    Abi.macosX64 || Abi.macosArm64 => "macos/libisar.dylib",
+    Abi.windowsX64 || Abi.windowsArm64 => "windows/libisar.dll",
+    _ => null,
+  };
+  if (relativeLibPath == null) {
+    throw UnsupportedError(
+      "isar_community_flutter_libs bundles no Isar core for ${Abi.current()}",
+    );
+  }
+
+  await Isar.initializeIsarCore(
+    libraries: {
+      Abi.current(): _packageRoot(
+        "isar_community_flutter_libs",
+      ).resolve(relativeLibPath).toFilePath(),
+    },
+  );
+}
+
+Uri _packageRoot(String packageName) {
+  final configFile = File(".dart_tool/package_config.json");
+  if (!configFile.existsSync()) {
+    throw StateError(
+      "${configFile.path} not found; run `flutter pub get` before the tests",
+    );
+  }
+
+  final config =
+      jsonDecode(configFile.readAsStringSync()) as Map<String, dynamic>;
+  final package = (config["packages"] as List)
+      .cast<Map<String, dynamic>>()
+      .firstWhere(
+        (p) => p["name"] == packageName,
+        orElse: () => throw StateError("package $packageName not resolved"),
+      );
+
+  // rootUri may be relative to .dart_tool/ and carries no trailing slash;
+  // without one, resolve() would replace its last segment.
+  final rootUri = package["rootUri"] as String;
+  return configFile.absolute.parent.uri.resolve(
+    rootUri.endsWith("/") ? rootUri : "$rootUri/",
+  );
+}