fix: preserve structured quota failures over ACP - #78

Summary

• recognize ZCode's model_rate_limited cause code while honoring an explicit context.retryable: false
• convert non-retryable turn.failed events into an ACP RequestError instead of allowing the SDK to collapse them to a generic Internal error
• preserve a small machine-readable error payload (code, reason, status/provider codes, retryability, and retryAfterMs) without forwarding response headers or cookies
• add an end-to-end prompt-handler regression test based on the observed coding-plan quota failure

This deliberately leaves delayed retry scheduling to ACP clients/control planes. The bridge keeps the session resumable and provides the metadata those clients need to decide when to resume.

Closes #77

Test plan

• ./node_modules/.bin/tsc --noEmit
• ./node_modules/.bin/eslint .
• ./node_modules/.bin/tsc
• ./node_modules/.bin/vitest run — 52 files, 723 tests passed