feat(v3-ref-seller): implement sync_accounts + list_accounts with billing_entity write-only projection guard wired

[bokelley]

Context

The v3 reference seller has the Account.billing_entity JSON column but does not implement sync_accounts or list_accounts. Because of this, adcp.types.project_account_for_response (the 3.1 bank-details write-only guard) is never called — the protection is purely architectural, not enforced.

Refs:

• examples/v3_reference_seller/src/platform.py
• examples/v3_reference_seller/src/models.py (Account.billing_entity)
• adcp.types.project_account_for_response

Acceptance Criteria

• sync_accounts implemented — persists incoming accounts (including billing_entity with bank details), then returns response via project_account_for_response
• list_accounts implemented — returns accounts projected through project_account_for_response so bank details are stripped
• Smoke test asserts bank_details (or any write-only field) never appears in either response
• Smoke test asserts non-write-only fields (display name, address, tax id format) DO appear

[bokelley]

Triage

Classification: Feature request
Bucket(s): examples
Status: drafting-pr
Milestone: (omitted — no milestone on file)

What the experts said:

• dx-expert: Non-blocking feature; natural key derivation needs adopter-customization notice (fixed); dry_run comment needs SQLAlchemy rollback sketch (fixed); blocker on bank-stripping helper choice explanation (fixed)
• docs-expert: Mock-based tests consistent with existing no-PG philosophy; assert on model_dump(mode="json") not raw Pydantic attr; module docstring should list new methods (done)
• code-reviewer (pre-PR): Three blockers fixed — missing name in sync response dict; sandbox None→False inconsistency between DB and wire; dry_run=True echoed back when session committed (lie on wire, fixed to always return False)

My take: Straightforward non-breaking addition to the examples bucket: two methods + two smoke tests, all wired to the already-merged write-only guard. Draft PR #403 covers all four acceptance criteria.

PR: #403 — feat(examples): sync_accounts + list_accounts with billing_entity write-only guard wired

---

Triaged by Claude Code. Session: https://claude.ai/code/session_01N51oUL2dxvvaVqe2xcjTuf

---

Generated by Claude Code