fix(core): preserve queued messages after recall

[AaronZ345]

Summary

• preserve messages queued after an active message is recalled
• resume the oldest surviving message in a replacement agent session while retaining FIFO order
• avoid treating an intentionally closed agent event channel as a crash that drops the queue

Root cause

The silent recall path cleared the entire pendingMessages queue when stopping the recalled turn. Messages sent after the recalled message were therefore accepted and acknowledged, then discarded without a response.

Tests

• go test -race ./core -run "TestCUJ_A8_RecallPreservesLaterQueuedMessage|TestHandleMessageRecallPreservesLaterQueueWhenAgentChannelCloses" -count=1
• go test ./core -count=1
• go test -tags no_web ./... -count=1

Maintenance

• Supersedes fix(core): preserve queued messages after recall #1569 after the 30-day maintenance refresh.
• Carries the same patch on the latest main.