Feature/ccm 15186 queue metrics

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

[Copilot]

Pull request overview

This PR introduces queue-size delta metrics for the update-letter-queue Lambda (used to maintain the pending letters queue table from the letters change stream), alongside dependency/lockfile updates.

Changes:

• Emit CloudWatch EMF metrics for per-supplier pending-queue deltas (QueueDelta) from update-letter-queue.
• Refactor queue update helpers to return numeric deltas (0/1) and aggregate per supplier across a batch.
• Update dependency locks/overrides (npm overrides for flatted/undici, and bump pyjwt in e2e Poetry lock).

Reviewed changes

Copilot reviewed 3 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/e2e-tests/poetry.lock	Updates locked pyjwt version for e2e test environment.
package.json	Adds npm overrides entries for flatted and undici.
package-lock.json	Updates resolved versions for flatted/undici per overrides.
lambdas/update-letter-queue/src/update-letter-queue.ts	Adds per-supplier queue delta metric aggregation and emission.
lambdas/update-letter-queue/src/tests/update-letter-queue.test.ts	Updates/extends unit tests to assert QueueDelta metric behavior and new edge-case handling.

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