fix(session): restore busy route handling and add regression coverage

[kitlangton]

Summary

• restore the session route contract for busy sessions by returning 400 instead of a generic 500
• add regression coverage for session abort/delete routes, subtask cancellation finalization, and the compaction parent invariant
• trim small follow-up cleanup in session helpers by removing dead revert state and redundant generic casts

Verification

• bun typecheck
• bun run test test/session/prompt-effect.test.ts
• bun run test test/session/compaction.test.ts
• bun run test test/server/session-actions.test.ts test/server/session-list.test.ts test/server/session-messages.test.ts test/server/session-select.test.ts