feat(examples): add 4 spec-required sales-* methods to v3 reference seller

[bokelley]

Closes #376

Adds the four sales-non-guaranteed specialism methods that validate_platform() will require in v6.0 rc.1. The v3 reference seller previously declared the specialism but only implemented the five core methods; this PR completes the surface so adopters forking it won't fail the boot check.

What changed

examples/v3_reference_seller/src/models.py — new PerformanceFeedback SQLAlchemy model (mirrors the MediaBuy idempotency-key pattern): tenant_id / account_id FKs, media_buy_id, idempotency_key (unique per tenant via perf_feedback_idem_uk), perf fields (performance_index, measurement_period JSON, metric_type, package_id, creative_id, feedback_source), received_at / created_at timestamps.

examples/v3_reference_seller/src/platform.py — four new async methods on V3ReferenceSeller:

• get_media_buys — queries existing MediaBuy table (tenant + account scoped), supports optional media_buy_ids / status_filter narrowing. Rows missing wire-required total_budget or currency are skipped with a logger.warning (production adopters should enforce budget at creation time).
• provide_performance_feedback — inserts a PerformanceFeedbackRow; DB-level idempotency via perf_feedback_idem_uk. Returns ProvidePerformanceFeedbackSuccessResponse(success=True).
• list_creative_formats — stub returning empty catalog. Extend for production by querying a CreativeFormat table or fetching from a creative management platform.
• list_creatives — stub returning empty list. Full persistence (wiring sync_creatives → Creative table) is deferred to a follow-up.

examples/v3_reference_seller/tests/test_smoke.py — four new tests: all-nine-methods presence check, list_creative_formats response validity, list_creatives response validity, and perf_feedback_idem_uk constraint assertion.

Nits (not fixed — noted for reviewers)

• list_creatives returning empty is intentional; wiring sync_creatives persistence is a separate concern.
• PerformanceFeedback.performance_index is nullable in the ORM (defensive for schema evolution) but always populated on insert from the required wire field.
• No Alembic migration included — reference seller uses docker-compose CREATE TABLE flow per README.

What tested

• ruff check examples/v3_reference_seller/ — clean
• pytest examples/v3_reference_seller/tests/ -v — 11 passed (7 pre-existing + 4 new)
• pytest tests/ -q --ignore=tests/conformance — 2715 passed, 18 skipped (no regressions)

Pre-PR review

• code-reviewer: approved — no blockers; stale "five"→"six" docstring nit fixed before opening
• dx-expert: approved after logger.warning fix for silent-skip path in get_media_buys

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_01R5JrFxdiswxFhwvEr3SWAd

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