swarm await_members redundantly fetches full history for reported members

Summary

swarm await_members performs an unnecessary CommReadContext request for every done member even when the await response already contains that member's retained completion report.

The formatter prefers AwaitedMemberStatus.completion_report over the separately fetched history fallback. Therefore, for the common case where a worker submitted or produced a retained completion report, the extra request transfers the member's full conversation history but cannot affect the rendered result.

Current behavior

For each done member, fetch_awaited_member_reports:

1. sends one CommReadContext request;
2. transfers that member's conversation history;
3. extracts the latest assistant response;
4. passes it beside the retained completion_report;
5. has the formatter ignore the history value because the retained report is present.

This scales as one extra socket round trip and one full-history transfer per completed worker.

Expected behavior

Use conversation history only as a compatibility/fallback path when a done member has no retained completion report on the await response. Preserve existing behavior for legacy or report-less completions.

Proposed change

Filter the backfill candidates to members where:

• done == true; and
• completion_report.is_none().

No protocol or rendered-output change is intended.

Validation

Add a focused regression test covering:

• a done member with a retained report: no backfill;
• a done member without one: backfill;
• a running member: no backfill.

Start by locating fetch_awaited_member_reports and the formatter that prefers AwaitedMemberStatus.completion_report. Add a focused regression test for done members with and without retained reports, plus a running member; done means only report-less members are backfilled and rendered output remains unchanged.