Use common metrics code and fix bug

[stevebux]

Description

Context

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
• If I have used the 'skip-trivy-package' label I have done so responsibly and in the knowledge that this is being fixed as part of a separate ticket/PR.

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[Copilot]

Pull request overview

This PR refactors update-letter-queue Lambda metric emission to use the shared EMF metrics helper code, and updates the unit tests accordingly, aligning this Lambda’s metrics formatting with the rest of the repository.

Changes:

• Replaced the Lambda’s local EMF metric builder with buildEMFObject and MetricStatus from @internal/helpers.
• Updated metric logging calls to use MetricStatus.Success / MetricStatus.Failure.
• Updated unit tests to assert the new metric name values.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lambdas/update-letter-queue/src/update-letter-queue.ts	Switches metric construction to the shared @internal/helpers EMF builder and standardizes metric status names.
lambdas/update-letter-queue/src/tests/update-letter-queue.test.ts	Updates assertions to match the new MetricStatus metric names.

Comments suppressed due to low confidence (1)

lambdas/update-letter-queue/src/tests/update-letter-queue.test.ts:307

• These metric assertions only check the _aws.CloudWatchMetrics[].Metrics[] entries. To ensure the EMF payload is actually valid/emittable, also assert the returned object contains the top-level value field keyed by the metric name (e.g. [MetricStatus.Success]: count / [MetricStatus.Failure]: count).

            Metrics: [
              expect.objectContaining({
                Name: MetricStatus.Success,
                Value: count,
              }),

[Vlasis-Perdikidis]

just an issue with imports order