Expose base traffic remainder in member traffic status

[juanlenoves]

Summary

• add base_traffic_remainder to the Scan member traffic-status OpenAPI response
• populate it from the sequencer traffic state
• cover the field in the existing member traffic integration test
• document the observable API addition in upcoming release notes

Verification

• Static OpenAPI/handler/test consistency verified locally
• Full Scala test suite not run because sbt is unavailable in the local environment

Pull Request Checklist

Cluster Testing

• If a cluster test is required, comment /cluster_test on this PR to request it, and ping someone with access to the DA-internal system to approve it.
• If an upgrade test is required, comment /upgrade_test on this PR to request it, and ping someone with access to the DA-internal system to approve it.
• If a hard-migration test is required (from the latest release), comment /hdm_test on this PR to request it, and ping someone with access to the DA-internal system to approve it.
• If a logical synchronizer upgrade test is required (from canton-3.5), comment /lsu_test on this PR to request it, and ping someone with access to the DA-internal system to approve it.

PR Guidelines

• Included the externally observable API addition in release_notes_upcoming.rst.
• No frontend screenshot is required.

Merge Guidelines

• Commit message is suitable for squash merge.