CCM-12896: Use Generated Event Types and Validators in Lambdas

[gareth-allan]

Description

Following #138, we now have auto-generated TypeScript types and JS validator code for all digital-letters events that we have produced schemas for. This PR updates all TypeScript code that uses manually-created Zod schemas for our events to use our new types and validation functions. The main changes are:

• Updated the ttl-create-lambda to use the new MESHInboxMessageDownloaded and ItemEnqueued types
• Updated the ttl-handle-expiry-lambda to use the new MESHInboxMessageDownloaded and ItemDequeued types
• Updated the EventPublisher library to work with our new auto-generated types
• Updated the Playwright tests to use the new event types
• Removed the old, manually created, TTL item and CloudEvent types from the utils package

Context

Using auto-generated types based on our JSON schemas, which are the source of truth for the digital letters event definitions, means that our code will always be using the correct shape for events published.

Validation

Verified the integration tests are working as expected:

After the latest set of changes was deployed, I used CloudWatch to verify events were being processed as expected.

The ttl-create lambda adding an entry to the table in DynamoDB and then publishing the ItemEnqueued event:

The ttl-handle-expiry lambda detecting the deletion of the entry from DynamoDB and sending the ItemDequeued event:

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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Sensitive Information Declaration

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.