CCM-15297: Prevent MeshDownloader from publishing duplicate events

[lapenna-bjss]

Description

This PR ensures the MESH Downloader does not process or publish a message with the same meshMessageId more than once.

Context

Duplicate messages can occur internally due to retries or when a message is re-polled before it is removed from the MESH inbox.

Key changes

• Added S3 conditional write (IfNoneMatch='*') to detect duplicates and raise DocumentAlreadyExistsError
• Updated S3 key structure to include meshMessageId for guaranteed uniqueness
• Skip event publishing on duplicates while still acknowledging messages
• Added a mesh-duplicate-downloads CloudWatch metric
• Added retry logic with backoff and jitter to EventPublisher (python) for transient failures
• Updated unit and component tests to cover duplicate handling, retries, and new S3 key format

Duplicate message is not published, but is acknowledged:

Duplicate metric:

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.

---

Sensitive Information Declaration

To ensure the utmost confidentiality and protect your and others privacy, we kindly ask you to NOT including PII (Personal Identifiable Information) / PID (Personal Identifiable Data) or any other sensitive data in this PR (Pull Request) and the codebase changes. We will remove any PR that do contain any sensitive information. We really appreciate your cooperation in this matter.

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

[aidenvaines-cgi]

you only need to do that if concatinating strings etc

[nhsd-angel-pastor]

Jus wondering whether we want to add in the PII bucket the if-none-header match restriction to enforce putObject requests to have the header (https://docs.aws.amazon.com/AmazonS3/latest/userguide/conditional-writes-enforce.html)