fix: guard should_replace against losing completed_at on same-run

[cgoea]

Motivation

A finalized test result could lose its completed_at timestamp to a later, less-complete snapshot of the same run.

The cause is in should_replace. A leaf/cell counts as terminal (is_terminal()) as soon as its status is terminal, independent of completed_at. But multiple notify events for the same shared-entry run each re-fetch and re-derive the whole job list, and those snapshots can disagree on completed_at: a cell's own notify step sees its job still running (no completed_at for itself), while a later sibling cell's snapshot, fetched after this cell actually finished, captures it. When the earlier, incomplete snapshot is processed after the complete one (same run_id/attempt), both are "terminal by status", so the existing do-not-downgrade guards don't fire, and the incomplete update overwrites the good completed_at with None.

Technical Details

• Added one guard to both Variant.should_replace and RunLeaf.should_replace in therock_status_document.py: within the same run_id/attempt, an update that has no completed_at cannot replace one that does.
• Scope is narrow: only the same-run/attempt, both-terminal-by-status case. All other precedence (run_id, run_attempt, terminal-vs-nonterminal) is unchanged.
• No caller changes: should_replace is already the sole gate consulted by upsert_leaf.

Test Plan

• test_same_attempt_terminal_with_completed_at_beats_terminal_without - existing terminal leaf with completed_at is not replaced by a same-run/attempt terminal update with completed_at=None.
• test_same_attempt_terminal_without_completed_at_replaced_by_one_with - the reverse still replaces (a snapshot gains completed_at).
• test_variant_same_attempt_terminal_with_completed_at_not_downgraded - same guard on the Variant path.
• test_merge_does_not_lose_completed_at_from_later_incomplete_snapshot - end-to-end reproduction of the production race through _merge_variant_leaf.
• Ran the full scripts/receive_therock/tests/ suite.

Test Result

All tests pass.