v2: Implement typed WhereDocument contains/notContains support end-to-end

[oss-amikos]

Context

WhereDocument typed helpers are currently placeholders and not fully implemented end-to-end.

Scope

• Implement WhereDocument.contains(...) and WhereDocument.notContains(...).
• Ensure proper serialization and builder integration for get/query flows.
• Document local vs cloud compatibility differences where relevant.

Acceptance Criteria

• Typed document filter helpers are fully functional.
• Unit + integration coverage validates behavior and errors.

Related: #94