chore: adopt Dart 3.9 toolchain — honest floor + tall-style reformat (T-353)

Raise the declared minimums in pubspec.yaml to what our deps already
require: Flutter >=3.35.0 / Dart >=3.9.0 (was 3.19.0 / 3.5.0). alchemist
0.12 needs Flutter 3.32; Dart 3.9 first ships in Flutter 3.35, so 3.35 is
the binding floor. Pin the exact build toolchain in .fvmrc (Flutter
3.44.1).

Moving to the Dart 3.9 language level switches `dart format` to the new
"tall" style and enables two new lints. This commit is the resulting
mechanical churn, isolated from any behaviour change:
  - whole-tree `dart format` reformat (tall style)
  - `dart fix` for unnecessary_underscores + use_null_aware_elements

No runtime behaviour change; `make test` green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/builtin/cli_install/cli_install_extension_test.dart

@@ -47,22 +47,26 @@ void main() {
   });
 
   test('contributes the install command', () async {
-    await boot(CliInstaller(
-      resolvedExecutable: '${tmp.path}/gui/clide',
-      env: {'PATH': '${tmp.path}/bin'},
-      bundledClientCandidates: const [],
-      installDir: '${tmp.path}/bin',
-    ));
+    await boot(
+      CliInstaller(
+        resolvedExecutable: '${tmp.path}/gui/clide',
+        env: {'PATH': '${tmp.path}/bin'},
+        bundledClientCandidates: const [],
+        installDir: '${tmp.path}/bin',
+      ),
+    );
     expect(f.services.commands.get('clide.installCli'), isNotNull);
   });
 
   test('warns on activation when clide is missing from PATH', () async {
-    await boot(CliInstaller(
-      resolvedExecutable: '${tmp.path}/gui/clide',
-      env: {'PATH': '${tmp.path}/empty'},
-      bundledClientCandidates: const [],
-      installDir: '${tmp.path}/bin',
-    ));
+    await boot(
+      CliInstaller(
+        resolvedExecutable: '${tmp.path}/gui/clide',
+        env: {'PATH': '${tmp.path}/empty'},
+        bundledClientCandidates: const [],
+        installDir: '${tmp.path}/bin',
+      ),
+    );
     final notes = f.services.notify.active;
     expect(notes, isNotEmpty);
     expect(notes.first.level, NotificationLevel.warning);
@@ -73,12 +77,7 @@ void main() {
     final gui = touchExec('${tmp.path}/gui/clide').path;
     final binDir = Directory('${tmp.path}/bin')..createSync();
     Link('${binDir.path}/clide').createSync(gui); // PATH clide → the GUI
-    await boot(CliInstaller(
-      resolvedExecutable: gui,
-      env: {'PATH': binDir.path},
-      bundledClientCandidates: const [],
-      installDir: binDir.path,
-    ));
+    await boot(CliInstaller(resolvedExecutable: gui, env: {'PATH': binDir.path}, bundledClientCandidates: const [], installDir: binDir.path));
     final notes = f.services.notify.active;
     expect(notes, isNotEmpty);
     expect(notes.first.level, NotificationLevel.warning);
@@ -89,12 +88,9 @@ void main() {
     final dev = touchExec('${tmp.path}/native/linux-x64/clide').path;
     final binDir = Directory('${tmp.path}/bin')..createSync();
     Link('${binDir.path}/clide').createSync(dev);
-    await boot(CliInstaller(
-      resolvedExecutable: '${tmp.path}/gui/clide',
-      env: {'PATH': binDir.path},
-      bundledClientCandidates: const [],
-      installDir: binDir.path,
-    ));
+    await boot(
+      CliInstaller(resolvedExecutable: '${tmp.path}/gui/clide', env: {'PATH': binDir.path}, bundledClientCandidates: const [], installDir: binDir.path),
+    );
     final notes = f.services.notify.active;
     expect(notes, isNotEmpty);
     expect(notes.first.level, NotificationLevel.info);
@@ -104,47 +100,35 @@ void main() {
   test('does not warn when clide is already installed', () async {
     final binDir = Directory('${tmp.path}/bin')..createSync();
     touchExec('${binDir.path}/clide');
-    await boot(CliInstaller(
-      resolvedExecutable: '${tmp.path}/gui/clide',
-      env: {'PATH': binDir.path},
-      bundledClientCandidates: const [],
-      installDir: binDir.path,
-    ));
+    await boot(
+      CliInstaller(resolvedExecutable: '${tmp.path}/gui/clide', env: {'PATH': binDir.path}, bundledClientCandidates: const [], installDir: binDir.path),
+    );
     expect(f.services.notify.active, isEmpty);
   });
 
   test('running the command installs the client and reports success', () async {
     final src = touchExec('${tmp.path}/bundle/clide-cli');
     final binDir = '${tmp.path}/bin';
-    await boot(CliInstaller(
-      resolvedExecutable: '${tmp.path}/gui/clide',
-      env: {'PATH': binDir},
-      bundledClientCandidates: [src.path],
-      installDir: binDir,
-    ));
+    await boot(CliInstaller(resolvedExecutable: '${tmp.path}/gui/clide', env: {'PATH': binDir}, bundledClientCandidates: [src.path], installDir: binDir));
     final r = await f.services.commands.execute('clide.installCli');
     expect(r.ok, isTrue);
     expect(r.data['installed'], '$binDir/clide');
     expect(File('$binDir/clide').existsSync(), isTrue);
-    expect(
-      f.services.notify.active.any((n) => n.level == NotificationLevel.success && n.title == 'clide CLI installed'),
-      isTrue,
-    );
+    expect(f.services.notify.active.any((n) => n.level == NotificationLevel.success && n.title == 'clide CLI installed'), isTrue);
   });
 
   test('running the command surfaces a tool error when nothing to install', () async {
-    await boot(CliInstaller(
-      resolvedExecutable: '${tmp.path}/gui/clide',
-      env: {'PATH': '${tmp.path}/bin'},
-      bundledClientCandidates: const [],
-      installDir: '${tmp.path}/bin',
-    ));
+    await boot(
+      CliInstaller(
+        resolvedExecutable: '${tmp.path}/gui/clide',
+        env: {'PATH': '${tmp.path}/bin'},
+        bundledClientCandidates: const [],
+        installDir: '${tmp.path}/bin',
+      ),
+    );
     final r = await f.services.commands.execute('clide.installCli');
     expect(r.ok, isFalse);
     expect(r.error!.kind, IpcErrorKind.toolError);
-    expect(
-      f.services.notify.active.any((n) => n.level == NotificationLevel.error),
-      isTrue,
-    );
+    expect(f.services.notify.active.any((n) => n.level == NotificationLevel.error), isTrue);
   });
 }