feat(examples): sync_accounts + list_accounts with billing_entity write-only guard wired

[bokelley]

Closes #377

Implements the two missing account-management methods on V3ReferenceSeller in the v3 reference seller example. The Account.billing_entity column already stores full bank details for invoicing — this PR wires the existing project_business_entity_for_response / project_account_for_response helpers so the write-only guard is enforced on every response path, not just architectural.

Changes

examples/v3_reference_seller/src/platform.py

• sync_accounts — upserts AccountRow (persists full billing_entity including bank); echoes billing_entity back with bank stripped via project_business_entity_for_response; returns SyncAccountsSuccessResponse. Natural upsert key is {brand.domain}:{operator} — comments explain the separator safety and tell adopters to replace this with their own stable key. dry_run is intentionally ignored (reference impl always commits); dry_run=False is returned to avoid lying to the caller about what was persisted.

• list_accounts — queries AccountRow scoped to the authenticated buyer agent with optional status/sandbox filters; reconstructs adcp.types.Account via model_validate (coerces billing_entity JSON dict → BusinessEntity Pydantic object, so project_account_for_response can call model_copy() safely); returns ListAccountsResponse.

Both methods re-query BuyerAgentRow by agent_url to resolve the FK — ctx.buyer_agent.ext doesn't carry the DB surrogate id because _row_to_agent in buyer_registry.py discards row.id. Comments at both call sites name the root cause.

examples/v3_reference_seller/tests/test_smoke.py

Two new async smoke tests using inline _StubSession fakes (consistent with the existing no-PG test philosophy). Both assert model_dump(mode="json") output so they test the serialised form, not just the Pydantic object attribute:

• test_sync_accounts_strips_bank_details — passes a billing_entity with IBAN/BIC; asserts "bank" absent and "legal_name" present in the response
• test_list_accounts_strips_bank_details — stubs an AccountRow with bank in billing_entity; asserts same

What was tested

• ruff check — clean (examples/ excluded from CI ruff run; verified explicitly)
• mypy examples/v3_reference_seller/src/platform.py --ignore-missing-imports — no new errors (3 pre-existing class-subclass errors in the file)
• pytest examples/v3_reference_seller/tests/ -v — 9/9 pass (2 new, 7 pre-existing)
• pytest tests/ -q — same baseline as main (1 pre-existing TLS test failing; unrelated to this PR)

Pre-PR review

• code-reviewer: approved after 3 blockers fixed — added "name" to sync response dict; fixed sandbox normalisation (None → False) to keep DB and wire consistent; changed dry_run echo from req.dry_run to False (was a lie on the wire when session committed)
• dx-expert: approved after 1 blocker fixed — expanded the natural-key comment to state adopters should replace it; added entity-level vs account-level helper guidance at sync_accounts call site; improved dry_run docstring with SQLAlchemy rollback sketch

Nits surfaced (not fixed — not blocking):

• get_products docstring doesn't end with "Production adopters…" (pre-existing inconsistency; out of scope)
• Update path preserves billing when acct.billing is None (intentional no-op; acceptable for reference impl)

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See adcp#3121
for context.

Session: https://claude.ai/code/session_01N51oUL2dxvvaVqe2xcjTuf

---

Generated by Claude Code