fix: treat already-fully-promoted target as success (idempotent)

[matthiasL-scality]

Summary

• When a workflow fails after a successful promote step, retrying the workflow caused the promote to always fail with `FAILED: target already fully promoted` (introduced in feat: allow resuming a partial promote without manual cleanup #223).
• Root cause: `.final_status` was copied as part of the regular batch — since S3 lists lexicographically, files starting with `.` arrive first, so `.final_status` could be written to the target before all other artifacts were copied. A promote interrupted after that point would appear complete but be incomplete.

This PR fixes both issues:

1. `.final_status` is now copied last, outside the main batch, as a reliable completion seal. Its presence on the target guarantees the promote finished in full.
2. A fully-promoted target is treated as idempotent success — if `.final_status` is already there, the promote returns `BUILD COPIED` immediately, so workflow retries are not blocked.

Fixes PTFE-3537.

Behaviour

Scenario	Before	After
Target empty	success	success (unchanged)
Target partially promoted (no `.final_status`)	resume	resume (unchanged)
Target fully promoted (`.final_status` present)	`FAILED`	`BUILD COPIED` (idempotent)
`.final_status` was a reliable seal	❌ could be written early	✅ always written last

Test plan

• `test_copy_idempotent_when_target_already_fully_promoted` — second promote on a complete target returns `BUILD COPIED`
• `test_resume_already_complete_promote` — same scenario from the resume angle
• `test_resume_promote_interrupted_before_final_status` — new test: target has all artifacts but no `.final_status`; promote completes successfully
• All existing tests (empty target, partial resume, multipart copy, cross-bucket, ingress) remain green

🤖 Generated with Claude Code

[claude]

LGTM

Review by Claude Code

[claude]

LGTM

Review by Claude Code