Fix broken pipe crash in task sessions

[renkun-ken]

Summary

• remove cancelled persistent sessions from the reusable worker pool immediately
• contain dispatch-time session failures so a closed command pipe cannot terminate the language server
• add regression coverage for the stale idle-state race and broken-pipe dispatch

Testing

• task tests: 35 passed
• affected integration groups (code actions, lintr, formatting): 200 passed
• full suite: 2,061 passed, with one unrelated parallel-only connection cleanup failure that passes standalone