test(#943): derive the schema numbers, and stop the cleanup masking failures

Three things that would have rotted:

- The downgrade tests hard-coded 3 as "a future schema". The day the real
  schema becomes 3 they would still pass while testing nothing, which is
  exactly when they are needed. Both now derive from this build's own
  schemaVersion. The message-rendering test deliberately keeps unrelated fixed
  numbers (7 and 6), so a wrong substitution shows up.
- Reading schemaVersion left an OffbandDatabase open for the whole file, and
  drift warns when two instances of one class are live. The probe moved into
  setUpAll and is closed immediately, because warnings in test output are how
  real ones get missed.
- The temp-directory cleanup threw a PathAccessException when the test failed,
  since a failure leaves the database open and Windows keeps the file locked.
  That replaced the actual failure with a cleanup error. Now swallowed.

Verified both downgrade tests fail when the `from > to` throw is disabled, and
that the three upgrade-direction guards stay green, so the group is pinned to
the behaviour it claims and not to the code's shape.

Task: meshcore-open-3e1
Agent Mail: TealGlen

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

test/storage/schema_downgrade_test.dart

@@ -27,6 +27,23 @@ void main() {
 
   tearDown(BlobStore.clearTestOverride);
 
+  /// This build's schema, and one past it. Derived rather than written as a
+  /// literal: hard-coding 3 here would turn these tests green-but-meaningless
+  /// the day the real schema becomes 3, which is exactly when they matter.
+  ///
+  /// Read in setUpAll and closed straight away, so the probe never overlaps
+  /// with the databases the tests build (drift warns about two live instances
+  /// of one class, and warnings in test output are how real ones get missed).
+  late final int current;
+  late final int future;
+
+  setUpAll(() async {
+    final probe = OffbandDatabase(NativeDatabase.memory());
+    current = probe.schemaVersion;
+    future = current + 1;
+    await probe.close();
+  });
+
   /// A database shaped like one a build at [userVersion] left behind, with
   /// both of today's tables and real data in the blob table.
   QueryExecutor databaseAtVersion(
@@ -78,7 +95,7 @@ void main() {
       // directions into onUpgrade ("Schema version upgrades and downgrades
       // will both be run here"). So whether a downgrade is caught is decided
       // by OUR onUpgrade, not by drift.
-      const downgrade = OpeningDetails(3, 2);
+      final downgrade = OpeningDetails(future, current);
       expect(downgrade.hadUpgrade, isTrue);
       expect(downgrade.wasCreated, isFalse);
     });
@@ -100,7 +117,7 @@ void main() {
   group('this build refuses a database from a future schema', () {
     test('opening one throws OffbandSchemaDowngrade', () async {
       final db = OffbandDatabase(
-        databaseAtVersion(3, {'contacts': '[{"name":"someone"}]'}),
+        databaseAtVersion(future, {'contacts': '[{"name":"someone"}]'}),
       );
       addTearDown(() async {
         try {
@@ -115,15 +132,18 @@ void main() {
     });
 
     test('and the message names both schemas and what to do', () {
+      // Deliberately not `current`/`future`: this checks the message renders
+      // whatever it is given, so fixed numbers unrelated to today's schema
+      // make a wrong substitution visible.
       const e = OffbandSchemaDowngrade(
-        from: 3,
-        to: 2,
+        from: 7,
+        to: 6,
         databasePath: r'C:\somewhere\offband_store.sqlite',
       );
       final text = e.toString();
 
-      expect(text, contains('schema 3'));
-      expect(text, contains('only understands 2'));
+      expect(text, contains('schema 7'));
+      expect(text, contains('only understands 6'));
       expect(text, contains('one-way'));
       expect(text, contains(r'C:\somewhere\offband_store.sqlite'));
     });
@@ -140,11 +160,18 @@ void main() {
       // the database's record of having been on a newer build. Before #943
       // this came back 2 and that evidence was gone.
       final dir = await Directory.systemTemp.createTemp('offband_downgrade');
-      addTearDown(() => dir.delete(recursive: true));
+      // Swallowed deliberately: if this test fails the database is left open,
+      // Windows keeps the file locked, and a throwing cleanup would replace
+      // the real failure with a PathAccessException.
+      addTearDown(() async {
+        try {
+          await dir.delete(recursive: true);
+        } catch (_) {}
+      });
       final file = File('${dir.path}/offband_store.sqlite');
 
       final db = OffbandDatabase(
-        databaseAtVersion(3, {'contacts': '[]'}, file: file),
+        databaseAtVersion(future, {'contacts': '[]'}, file: file),
       );
       await expectLater(
         BlobStore(db).read('contacts'),
@@ -156,14 +183,14 @@ void main() {
 
       // Read it back on a connection that declares no schema of its own.
       final raw = NativeDatabase(file);
-      final probe = _Probe(raw);
+      final probe = _Probe(raw, future);
       addTearDown(probe.close);
       expect(
         await probe
             .customSelect('PRAGMA user_version')
             .getSingle()
             .then((r) => r.data.values.first as int),
-        3,
+        future,
         reason: 'the refusal must leave the version alone',
       );
     });
@@ -185,7 +212,7 @@ void main() {
         addTearDown(db.close);
         await BlobStore(db).read('anything');
 
-        expect(await userVersionOf(db), 2);
+        expect(await userVersionOf(db), current);
       },
     );
 
@@ -211,7 +238,7 @@ void main() {
       addTearDown(db.close);
 
       expect(await BlobStore(db).read('contacts'), '[{"name":"keep me"}]');
-      expect(await userVersionOf(db), 2);
+      expect(await userVersionOf(db), current);
     });
   });
 }
@@ -219,11 +246,12 @@ void main() {
 /// A drift database with no tables and no migration, for reading a PRAGMA off
 /// a file without declaring a schema that would itself trigger a migration.
 class _Probe extends GeneratedDatabase {
-  _Probe(super.executor);
+  _Probe(super.executor, this.schemaVersion);
 
+  /// Matches the file's own version, so opening it runs no migration.
   @override
-  Iterable<TableInfo<Table, dynamic>> get allTables => const [];
+  final int schemaVersion;
 
   @override
-  int get schemaVersion => 3;
+  Iterable<TableInfo<Table, dynamic>> get allTables => const [];
 }