ci: opt-in data-server pairing for integration tests (sdc-pair)

[steiler]

Summary

• Integration tests now receive data-server main by default instead of the latest GitHub Release semver for the data-server image.
• Same-named branch on sdcio/data-server is used only when this PR has the label sdc-pair. The workflow resolves the branch tip to a short SHA (first 7 hex chars) to match ghcr.io/sdcio/data-server tags published by GoReleaser.
• workflow_dispatch: optional input dataserver_version overrides the data-server image tag when needed.
• pull-requests: read allows push runs to see whether an open PR on this branch has sdc-pair, matching pull_request behaviour.

How to use cross-repo (config-server + data-server)

1. Create the sdc-pair label in this repo if needed (Settings → Labels).
2. Use the same branch name on config-server and data-server.
3. Ensure data-server CI has pushed images for that branch’s tip (short SHA tag).
4. Add sdc-pair to this PR when integration should use the matching data-server branch; otherwise integration uses data-server main.

Related PR (data-server): sdcio/data-server#447

Test plan

• PR / push without sdc-pair uses data-server main in integration.
• PR with sdc-pair and matching branch on sdcio/data-server uses the expected short SHA tag when images exist.
• workflow_dispatch with dataserver_version overrides auto resolution.