test: cover _resolveDatabasePath retry logic to catch budget regressions (#167)

Issue #166 exposed that the retry budget in _resolveDatabasePath() was too
small for slow Android devices, but no unit test exercised that code path.

Expose two testing helpers (resolveDatabasePathForTesting /
resetDatabasePathForTesting) and add two new tests using fake_async:

- verifies the retry loop eventually succeeds after transient failures
  (a _SucceedAfterNPathProvider that fails N times then returns a path)
- verifies the function throws PlatformException with the right message
  after exhausting all retries

Both tests advance fake time instead of waiting real-world milliseconds,
so they run in < 1 ms each.

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

lib/data/db/database.dart

@@ -616,6 +616,11 @@ Future<String> _resolveDatabasePath() async {
   );
 }
 
+// These two functions are only called from unit tests (database_path_test.dart).
+// They expose internals that cannot be reached via the public API.
+Future<String> resolveDatabasePathForTesting() => _resolveDatabasePath();
+void resetDatabasePathForTesting() => _dbPath = null;
+
 LazyDatabase _openConnection() {
   return LazyDatabase(() async {
     final file = File(await _resolveDatabasePath());

test/unit/database_path_test.dart

@@ -1,3 +1,6 @@
+import 'dart:async';
+
+import 'package:fake_async/fake_async.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:path_provider_platform_interface/path_provider_platform_interface.dart';
@@ -19,6 +22,30 @@ class _UnavailablePathProvider extends Fake
   }
 }
 
+// Fake PathProviderPlatform that fails the first [failCount] calls, then
+// returns a fixed path.  Used to exercise the retry loop in
+// _resolveDatabasePath() without waiting for real timers.
+class _SucceedAfterNPathProvider extends Fake
+    with MockPlatformInterfaceMixin
+    implements PathProviderPlatform {
+  _SucceedAfterNPathProvider({required this.failCount});
+
+  final int failCount;
+  int _callCount = 0;
+
+  @override
+  Future<String?> getApplicationSupportPath() async {
+    _callCount++;
+    if (_callCount <= failCount) {
+      throw PlatformException(
+        code: 'channel-error',
+        message: 'Simulated: path_provider channel not ready',
+      );
+    }
+    return '/tmp/test_app_support';
+  }
+}
+
 void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
@@ -38,4 +65,69 @@ void main() {
       await expectLater(initDatabasePath(), completes);
     },
   );
+
+  // Tests for _resolveDatabasePath() — the lazy retry path called on first DB
+  // access when initDatabasePath() already failed.  fake_async lets us advance
+  // the back-off timers without waiting real-world milliseconds.
+
+  test(
+    '_resolveDatabasePath retries and eventually succeeds after transient failures',
+    () {
+      resetDatabasePathForTesting();
+      final prev = PathProviderPlatform.instance;
+      // Fail 3 times, succeed on the 4th call.  The delays in
+      // _resolveDatabasePath are [200, 500, 1000, 2000, 4000] ms, so three
+      // failures cost 200+500+1000 = 1700 ms before the fourth attempt.
+      PathProviderPlatform.instance = _SucceedAfterNPathProvider(failCount: 3);
+      addTearDown(() {
+        PathProviderPlatform.instance = prev;
+        resetDatabasePathForTesting();
+      });
+
+      fakeAsync((fake) {
+        String? result;
+        unawaited(resolveDatabasePathForTesting().then((r) => result = r));
+
+        // Advance fake time through the three back-off delays.
+        fake.elapse(const Duration(milliseconds: 200 + 500 + 1000 + 1));
+
+        expect(result, isNotNull);
+        expect(result, endsWith('sharedinbox.db'));
+      });
+    },
+  );
+
+  test(
+    '_resolveDatabasePath throws PlatformException after exhausting all retries',
+    () {
+      resetDatabasePathForTesting();
+      final prev = PathProviderPlatform.instance;
+      PathProviderPlatform.instance = _UnavailablePathProvider();
+      addTearDown(() {
+        PathProviderPlatform.instance = prev;
+        resetDatabasePathForTesting();
+      });
+
+      fakeAsync((fake) {
+        Object? caughtError;
+        unawaited(
+          resolveDatabasePathForTesting().catchError((Object e) {
+            caughtError = e;
+            return ''; // ignored; satisfies the Future<String> return type
+          }),
+        );
+
+        // Advance past all five back-off delays: 200+500+1000+2000+4000 ms.
+        fake.elapse(
+          const Duration(milliseconds: 200 + 500 + 1000 + 2000 + 4000 + 1),
+        );
+
+        expect(caughtError, isA<PlatformException>());
+        expect(
+          (caughtError! as PlatformException).message,
+          contains('cannot open database'),
+        );
+      });
+    },
+  );
 }