fix(dart/templates): tolerant int deserialization in models; make \$s…

[sadji-zigadi]

What does this PR do?

• Fixes Dart model deserialization for integer fields.
• For Dart templates, parses ints as (map['$field'] as num?)?.toInt() instead of assigning raw values.
• Special-cases "$sequence" to be nullable in generated models to tolerate missing values.

Why:

• Some servers return "$sequence" as a string or omit it. Previous code caused String → int and Null → int runtime errors in generated SDKs.

Test Plan

• Regenerated the Dart/Flutter SDK from this template.
• Verified generated Document.fromMap contains:
  • $sequence: (map['\$sequence'] as num?)?.toInt(), (nullable)
  • Other required ints use ?? 0 default when applicable.
• Ran:
  • dart format .
  • dart analyze
  • flutter test on generated SDK
• Manual check: Created documents and listed documents with/without "$sequence" and with "$sequence": "3"; confirmed no crashes and correct values.

Related PRs and Issues

• 🐛 Bug Report: TypeError when deserializing $sequence field from String to int sdk-for-flutter#261
• 🐛 Bug Report: Breaking Change in Appwrite SDK: type 'Null' is not a subtype of type 'int' in lisiting documents appwrite#10253

Have you read the Contributing Guidelines on issues?

Yes.