fix(flutter/test): isVersionNewer date-style assertion was wrong

pub_semver is permissive with leading zeros — '2026.05.10' parses as
2026.5.10, so date-style tags get strict semver ordering, not the
unparseable-fallback path. The test was asserting "any difference =
newer" for date strings, which is incorrect; the actual behavior
(and the right behavior) is proper ordering.

Split the test group into two:
- date-style (parses as semver): newer/older/equal assertions
- truly unparseable (e.g. branch names like 'main', 'dev'): the
  fallback's "any difference = newer" semantics still apply

Implementation in client_update_provider.dart unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/test/update/client_update_provider_test.dart

@@ -31,18 +31,31 @@ void main() {
     });
   });
 
-  group('isVersionNewer (non-semver fallback)', () {
-    test('different strings → newer', () {
-      // Date-tag-style versions don't parse as semver; falls back to
-      // string inequality. Operator can dismiss if the comparison is
-      // wrong (the alternative — silent skip — would mean operators
-      // never see updates).
+  group('isVersionNewer (date-style versions parse as semver)', () {
+    // pub_semver is permissive with leading zeros — '2026.05.10'
+    // parses as 2026.5.10. So date-style tags get strict ordering,
+    // not the unparseable-fallback path.
+    test('newer date → newer', () {
       expect(isVersionNewer('2026.05.10', '2026.05.09'), isTrue);
-      expect(isVersionNewer('2026.05.09', '2026.05.10'), isTrue);
     });
-
-    test('equal strings → not newer', () {
+    test('older date → not newer', () {
+      expect(isVersionNewer('2026.05.09', '2026.05.10'), isFalse);
+    });
+    test('equal date → not newer', () {
       expect(isVersionNewer('2026.05.10', '2026.05.10'), isFalse);
     });
   });
+
+  group('isVersionNewer (truly non-semver fallback)', () {
+    // Branch-name-style strings (no version structure) hit the
+    // try/catch fallback. Any string difference reads as "newer" so
+    // operators see _something_ rather than silently miss updates.
+    test('different unparseable strings → newer', () {
+      expect(isVersionNewer('main', 'dev'), isTrue);
+      expect(isVersionNewer('dev', 'main'), isTrue);
+    });
+    test('equal unparseable strings → not newer', () {
+      expect(isVersionNewer('main', 'main'), isFalse);
+    });
+  });
 }