fix(models): tolerate $sequence int/string/null in Document.fromMap

[sadji-zigadi]

What does this PR do?

• Fixes a deserialization crash when "$sequence" is returned as a string or omitted.
• Changes Document.$sequence to nullable (int?) and parses with:
  • $sequence: (map['\$sequence'] as num?)?.toInt(),
• Prevents Null → int and String → int errors during fromMap.

Test Plan

• Added/updated unit tests:
  • String "$sequence": "3" → doc.$sequence == 3
  • Missing "$sequence" → doc.$sequence == null
• Ran:
  • dart format .
  • dart analyze
  • flutter test

Related PRs and Issues

• appwrite/sdk-for-flutter#261
• appwrite/appwrite#10253

Notes

• This makes the field nullable, which is technically a public API change but aligns with real-world server responses (string or absent).
• If you prefer to keep the field non-nullable, I can switch to:
  • final int $sequence;
  • fromMap: $sequence: (map['\$sequence'] as num?)?.toInt() ?? 0,
    which preserves the API and defaults missing values to 0.

Have you read the Contributing Guidelines on issues?

Yes.

[github-actions]

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