chore(gossipsub): Check whether published partials are stale

[dknopik]

Description

Application-side race conditions might cause calls to publish_partial to arrive in an unintended order. Currently, gossipsub silently accepts partial and overwrites the previously cached value - causing stale data to be replied on incoming requests.

This PR uses Metadata::update to compare the cached with the newly published value, and only proceeds with the publish if there is any fresh data.

AI Assistance Disclosure

Tools used (required — write none if no AI was used): Claude wrote the test
Attestation (required):

• I have read every line of this diff, understand what it does, and can explain it in review.

Notes & open questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates