Maintenance: Resolve Ty diagnostics for event_handler module

[ConnorKirk]

Why is this needed?

This is a sub-issue of #7902.

The event_handler/ module has 85 ty diagnostics - the largest of any module. Due to its size, this will require a dedicated effort and potentially multiple PRs to resolve.

Which area does this relate to?

• Static typing
• Event Handler - REST API
• Event Handler - GraphQL API

Solution

Resolve all 85 ty diagnostics in event_handler/ and remove from ty exclusion list in pyproject.toml. Given the scope, consider:

1. Grouping fixes by sub-module or diagnostic type
2. Splitting into multiple PRs if needed
3. Prioritising fixes that unblock other work

Acknowledgment

• This request meets Powertools for AWS Lambda (Python) Tenets
• Should this be considered in other Powertools for AWS Lambda languages? i.e. Java, TypeScript, and .NET