SPRXCLT-24: add putUserMetadata method

[jonathan-gramain]

Summary

• Add putUserMetadata(key, metadata, reqUids, callback) method that updates an object's user metadata in-place by sending a zero-byte PUT with x-scal-cmd: update-usermd, without replacing the object's content
• Add unit test covering the new method: verifies both that the metadata is updated and that the existing object content is preserved
• Update the unit test server handler to simulate the x-scal-cmd: update-usermd sproxyd behaviour (metadata-only update, content body untouched)

Test plan

• npm test passes (85 tests)
• New test runs across all four client variants (immutable, non-immutable, with and without failover)

🤖 Generated with Claude Code

[bert-e]

Hello jonathan-gramain,

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[bert-e]

Waiting for approval

The following approvals are needed before I can proceed with the merge:

• the author

• 2 peers

[jonathan-gramain]

/approve

[bert-e]

I have successfully merged the changeset of this pull request
into targetted development branches:

• ✔️ development/9

The following branches have NOT changed:

• development/6.4
• development/7.10
• development/7.4
• development/8.0
• development/8.1
• development/8.2

Please check the status of the associated issue SPRXCLT-24.

Goodbye jonathan-gramain.

The following options are set: approve