[PM-4252] Change attachment Size to be represented as a string

[Hinton]

Type of change

- [x] Bug fix
- [ ] New feature development
- [ ] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
- [ ] Build/deploy pipeline (DevOps)
- [ ] Other

Objective

We use JsonNumberHandling.WriteAsString) for Size in AttachmentResponse, unfortunately this isn't understood by the Open API generation we use and the generated rust code attempts to cast int to string which is an invalid operation.

Changes it to be an actual string in code, which should functionality be equivalent to existing behavior.

Before you submit

• Please check for formatting errors (dotnet format --verify-no-changes) (required)
• If making database changes - make sure you also update Entity Framework queries and/or migrations
• Please add unit tests where it makes sense to do so (encouraged but not required)
• If this change requires a documentation update - notify the documentation team
• If this change has particular deployment requirements - notify the DevOps team

[bitwarden-bot]

Checkmarx One – Scan Summary & Details – 27296972-604f-4228-96d1-862e1b4e80c3

No New Or Fixed Issues Found