bug(v3-ref-seller): storyboard terminal-error codes mismatch on three update / create scenarios

[bokelley]

Surfaced by #693 (which made the storyboard CI gate actually assert on results).

Failures

The AdCP storyboard runner — v3 reference seller (translator) job fails three scenarios with terminal-code mismatches at /adcp_error/code:

1. media_buy_seller/measurement_terms_rejected/create_media_buy_aggressive_terms — storyboard expects TERMS_REJECTED
2. media_buy_seller/invalid_transitions/update_unknown_media_buy — storyboard expects MEDIA_BUY_NOT_FOUND
3. media_buy_seller/invalid_transitions/update_unknown_package — storyboard expects PACKAGE_NOT_FOUND

Each is a terminal code that cascade rules can't mask — the seller is returning some other error code in adcp_error.code (or returning success). These are real spec-conformance regressions in the v3 reference seller, not storyboard fixture issues.

Run

ci run 25732467897, job 75560889145

Full step output:

- create_media_buy_aggressive_terms [media_buy]: Error code is TERMS_REJECTED at /adcp_error/code
- update_unknown_media_buy [media_buy]: Error code is MEDIA_BUY_NOT_FOUND at /adcp_error/code
- update_unknown_package [media_buy]: Error code is PACKAGE_NOT_FOUND at /adcp_error/code

What to investigate

For each failure: pull the request/response from the storyboard run artifact (v3-storyboard-result-1.zip), confirm what code the seller actually returns, then map that to either:

• a seller-side bug: handler returns the wrong code → fix handler
• a translator-side bug: upstream code translation dropping/remapping the wrong code → fix the translation map
• a spec-vs-code drift: the canonical AdCP error-code enum changed and the seller hasn't caught up → bump

Why these block #693

#693's whole purpose is to make the storyboard gate assert on results. Until these three are resolved, the v3 storyboard CI job stays red, blocking #693 from landing or making main permanently red on merge.

Acceptance

• All three storyboard steps pass with the spec-correct error code in adcp_error.code
• Root cause documented (seller / translator / spec drift) per step in the fix PR
• __spec_conformance__/account_discovery/list_or_sync_accounts is a separate failure cluster tracked in feat(v3-ref-seller): implement sync_accounts + list_accounts with billing_entity write-only projection guard wired #377 — not in scope here