Add required per-turn output schemas

[hunterassembly]

Summary

• propagate a per-turn required output-schema flag across the Eve client, channel, durable session, and workflow boundaries
• force exactly one final-output step when a required structured turn would otherwise terminate as prose
• expose the terminal step.failed event to channels so consumers can settle required-output failures truthfully

Proof

• pnpm --filter eve test:unit — 565 files, 5,890 passed, 1 skipped
• pnpm --filter eve typecheck
• pnpm --filter eve build
• autoreview: clean after one prior session-token-limit repair

This is the framework dependency for Remi REMI-1525.

[vercel]

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[vercel]

The continue path accepts outputSchemaRequired: true without an accompanying outputSchema but silently drops the flag, so required-final-output enforcement never triggers and no error is surfaced.