Support weaker consistency model for S3 MPUs

[DaveCTurner]

Adjusts the implementation of linearizable registers in S3 repositories
to allow for the weaker multipart upload API semantics observed in
practice.

Also adjusts the S3 test fixture to (optionally) simulate the weaker
semantics, and extends the repository analysis REST tests to cover both
cases.

[elasticsearchmachine]

Pinging @elastic/es-distributed-coordination (Team:Distributed Coordination)

[elasticsearchmachine]

Hi @DaveCTurner, I've created a changelog YAML for you.

[github-actions]

🔍 Preview links for changed docs

• docs/release-notes/known-issues.md

[github-actions]

ℹ️ Important: Docs version tagging

👋 Thanks for updating the docs! Just a friendly reminder that our docs are now cumulative. This means all 9.x versions are documented on the same page and published off of the main branch, instead of creating separate pages for each minor version.

We use applies_to tags to mark version-specific features and changes.

Expand for a quick overview

When to use applies_to tags:

✅ At the page level to indicate which products/deployments the content applies to (mandatory)
✅ When features change state (e.g. preview, ga) in a specific version
✅ When availability differs across deployments and environments

What NOT to do:

❌ Don't remove or replace information that applies to an older version
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❌ Don't forget that applies_to tags can be used at the page, section, and inline level

🤔 Need help?

• Check out the cumulative docs guidelines
• Reach out in the #docs Slack channel