CCM-13519 Add idempotency layer in front of upsert

[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

DT3-Specific Checklist

• If I have added a new resource (SQS, Lambda, Gateway, DDB table, etc), I have created the appropriate alarms

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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.

[Copilot]

Pull request overview

Adds an AWS Lambda Powertools idempotency layer to the upsert-letter Lambda so repeated deliveries of the same event can be safely deduplicated using a dedicated DynamoDB table.

Changes:

• Introduces Powertools idempotency (makeIdempotent + DynamoDB persistence layer) around per-record upsert processing.
• Adds a new DynamoDB table and Terraform wiring (env var + IAM) to persist idempotency records.
• Updates tests/fixtures and package manifests to support the new idempotency behavior and infra.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
tests/helpers/event-fixtures.ts	Adjusts prepared event fixture generation (random IDs + override spreading).
package.json	Adds Powertools idempotency dependency at repo root (in addition to workspace).
package-lock.json	Locks Powertools idempotency and transitive dependencies.
lambdas/upsert-letter/src/handler/upsert-handler.ts	Wraps record processing with Powertools idempotency, adds context handling.
lambdas/upsert-letter/src/handler/tests/upsert-handler.test.ts	Adds idempotency mocking and a replay-focused unit test.
lambdas/upsert-letter/src/config/env.ts	Adds required env var for idempotency table name.
lambdas/upsert-letter/src/config/deps.ts	Adds DynamoDB persistence layer to dependency container.
lambdas/upsert-letter/src/config/tests/env.test.ts	Updates env var loading test to include new required var.
lambdas/upsert-letter/package.json	Adds Powertools idempotency dependency to the lambda workspace package.
infrastructure/terraform/components/api/module_lambda_upsert_letter.tf	Passes idempotency table name to Lambda and grants DynamoDB access.
infrastructure/terraform/components/api/ddb_table_upsert_idempotency.tf	Creates DynamoDB table used by the idempotency persistence layer.