fix(server): preserve completion cursors when deleting rollouts

[ningyixi]

Deleting a completed rollout leaves its ID in the completion log. Re-enqueueing the same client-assigned ID then makes /api/rollouts/terminal return a queuing rollout; completing it adds a second entry for the same new rollout.

Replace deleted completion entries with tombstones, keeping the existing index-based cursors stable. A live ID-to-index map keeps deletion constant-time instead of rescanning the growing completion log. Wire response fields and pagination semantics stay the same.

Related: #547. That PR removes entries from the list, shifting positions underneath existing cursors. This alternative preserves those positions; its API regression also verifies that a cursor obtained before deletion still returns the next surviving completion. It covers recreation, exclusion before completion, and exactly one new completion after recreation.

Validation:

• The regression fails on current main: a newly queued rollout appears in the terminal page.
• uv run --locked --no-sync pytest -q tests/server — 33 passed.
• Repository-wide Ruff lint, formatting checks, pre-commit hooks, and copyright-header checks passed.
• Pyright passed for all four changed files. The full VERL/CPU training suite was not run.

Prepared with Codex AI assistance; the diff and before/after regression results were checked locally. No model service or GPU is needed for these tests.

[Copilot]

🟢 Approval recommended

The focused changes preserve cursor positions and prevent stale ID replay, with regression coverage and no identified blocking issues.

No open issues 🎉

What changed in this PR

Fixes deleted rollout IDs reappearing in terminal results while preserving existing pagination cursors, avoiding the index shifts proposed in #547.

Changes:

• Uses tombstones and an ID-to-index map for constant-time deletion without shifting cursors.
• Adds regression coverage for deletion, ID reuse, and cursor stability.

File	Description
tests/​server/​test_endpoints.py	Tests deletion, recreation, and stable completion cursors.
tests/​server/​conftest.py	Clears the new index between tests.
agentlightning/​server/​store.py	Defines tombstone-capable storage and the completion index.
agentlightning/​server/​routes/​rollouts.py	Maintains the index and skips deleted completion entries.

_{Review effort: Balanced}

[ningyixi]

@microsoft-github-policy-service agree