fix(build): handle line breaks after class names in model dictionary …

[mastermakrela]

• Fixes #662 where model dictionary builder fails when Flutter formatter breaks lines after long class names
• Replaces fragile string matching with robust regex pattern that handles whitespace including newlines
• Prevents build failures for projects with class names exceeding 80-character limit

The original code used entry.value.contains('class ${annotation.element.name} ') which expected a space after class names. When Flutter's formatter formatted long declarations like:

class VeryLongClassNameThatExceedsTheEightyCharacterLimit
    extends SomeBaseClass {

The space was replaced with a newline, causing the match to fail and breaking the build with "No element" errors.

Changes:

• Updated packages/brick_build/lib/src/builders/model_dictionary_builder.dart:67
• Changed from string match to RegExp(r'class\s+${annotation.element.name}\s*')
• Now correctly handles spaces, tabs, and newlines after class names

Testing:

• Verified regex matches all class declaration formats (with/without line breaks)
• Tested with various whitespace combinations
• Maintains backward compatibility with existing code