Make generated Dart and Flutter method examples analyze cleanly #2091

Tracks PR #2090.

Plan

Goal: Generated Dart and Flutter method examples should pass dart analyze, and CI should keep them that way.

What changes:

• templates/dart/docs/example.md.twig and templates/flutter/docs/example.md.twig: drop the permission.dart / role.dart imports (they are part of the main library, so analyze reports import_of_non_library), import dart:typed_data for Dart location examples, drop the unused dart:io import from upload examples (Flutter keeps it for location examples), and stop rendering models. result = / models.Any result = for methods with no response model or an any response (await without a typed result, or use dynamic).
• New scripts/validate-dart-examples.sh: pulls the code block out of every generated docs example, wraps it in main(), and runs dart analyze --fatal-warnings inside the generated package.
• .github/workflows/validation.yml: the dart and flutter validation jobs run that script after analyzing the SDK, like validate-go-examples.sh does for Go.

Remaining steps: None. The linked PR is the full change.

Done when:

• fix(dart): make generated method examples analyze cleanly #2090 is merged
• The dart and flutter validation jobs pass with the example check enabled