Fix DSH retired Host turn fences

[xianjianlf2]

This stacked PR fixes the latest DeepSeek Harness review issue from milind-soni#407.

Changes:

• Retire/fence the Host turn before queue submission overflow cancellation.
• Retire/fence observed Host turns before stream-loss local failure releases the active slot.
• Preserve existing unnumbered Host terminal behavior by fencing stream-loss only when a Host turn number was observed.
• Add regressions for queue-overflow retry and stream-loss retry so delayed old turn/end frames cannot complete the replacement turn.

Validation:

• pnpm exec vitest run server/drivers/deepseek-harness/deepseek-harness.test.ts
• pnpm exec vitest run server/drivers/deepseek-harness/*.test.ts server/deepseek-harness-settings.test.ts
• pnpm run typecheck