chore(repo): enable migrate_design_widgets to guard the migration

Without the lint nothing stops a new file importing
package:flutter/material.dart, silently reintroducing the split between
Flutter's Material types and material_ui's.

Enabling it is safe at the 3.44 floor even though that analyzer does not
recognise the rule. `undefined_lint` is reported against
analysis_options.yaml itself, so it only surfaces when that file is
inside the analysed directory. legacy_version_analyze runs
`cd packages/X/lib && dart analyze --fatal-infos .`, which leaves the
options file out of scope: verified exit 0 from lib/ versus exit 2 from
the package root with the same unknown rule enabled. The main analyze
job runs the latest stable, which knows the rule.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: xsahil03x

analysis_options.yaml

@@ -154,4 +154,9 @@ linter:
     - unnecessary_overrides
     - prefer_null_aware_method_calls
     - use_named_constants
-    - use_raw_strings
+    - use_raw_strings
+    # Guards the material_ui/cupertino_ui migration: fails any new import
+    # of package:flutter/material.dart or package:flutter/cupertino.dart.
+    # Unknown to the 3.44 floor, but legacy_version_analyze analyses lib/
+    # rather than the package root, so it never sees this file.
+    - migrate_design_widgets