chore: clean analyzer to zero issues

Whole-project `flutter analyze` exited 1 on 17 pre-existing info/warning
lints (no errors) in dev tools and test files. Clean them so analyze is
green:
- editor_repository_test: drop the redundant sqflite_common import; keep
  the used utils import with a transitive-dep ignore.
- pen_* widget tests: `(_, __)` wildcard params -> `(_, _)`.
- search_snippet_test / gen_bench_pdf / gen_dense_strokes: drop needless
  interpolation braces; remove an unused `Size` show and an unused local;
  mark the gen tool as print-allowed.

No runtime behavior changed. flutter analyze: No issues found. Affected
tests: 22/22 pass.

test/search_snippet_test.dart

@@ -53,7 +53,7 @@ void main() {
 
   test('the full match is included even when longer than the window', () {
     final long = 'x' * 100;
-    final s = snippetFor('pre ${long} post', long, window: 10)!;
+    final s = snippetFor('pre $long post', long, window: 10)!;
     expect(s.match, long);
     expect(s.matchLength, 100);
   });