fix(bridge): preserve typed pre-provider rejections

[drewstone]

Summary

• Preserve Router's exact error.provider_dispatch: "not_started" fact through Pi's message-only error path.
• Use a random request-scoped marker so only the matching Router response can carry the proof; missing or different values remain unknown.
• Keep the proof in bridge JSON and SSE errors for Runtime, and prevent host accounting from replacing it with a generic usage mismatch.

Verification

• pnpm typecheck
• pnpm exec vitest run tests/pi-provider-dispatch.test.ts tests/failure-attribution.test.ts — 26/26
• pnpm exec vitest run tests/pi-backend.test.ts — 58/58
• pnpm exec vitest run tests/pi-inference-isolation.test.ts -t "carries only Router" — 1/1
• pnpm exec vitest run tests/pi-inference-isolation.test.ts tests/failure-attribution.test.ts tests/pi-provider-dispatch.test.ts — 49/50 in isolation; the one failure is the existing real-MCP timing test, unrelated to this change.

Router contract source: merged Router #361, 26f62596d5093db74d5f22f1dc6516c723eeb13b.

No provider calls, credentials, ports, publish, or tag operations were used.