fix(types): align SyncCreativesResponse Creative & CreateMediaBuyResponse1 with rc.9 schema

[bokelley]

Two codegen type-accuracy fixes against the cached AdCP 3.1.0-rc.9 schemas. Both buggy classes are hand-authored snippets in scripts/post_generate_fixes.py, so a future schema regen would not auto-fix them — the snippets are edited and the generated files regenerated to match.

#918 — SyncCreativesResponse Creative missing 9 of 12 spec fields

The per-creative result item (oneOf[0].properties.creatives.items in sync-creatives-response.json, required=[creative_id, action]) defines 12 properties. The snippet declared only creative_id, action, errors. Added the 9 missing optional fields with types derived from the rc.9 bundled schema:

field	type
account	Account | None = None
status	CreativeStatus | str | None = None
platform_id	str | None = None
changes	list[str] | None = None
warnings	list[str] | None = None
preview_url	str | None = None
expires_at	AwareDatetime | None = None
assigned_to	list[str] | None = None
assignment_errors	dict[str, str] | None = None

#914 — CreateMediaBuyResponse1 incorrectly declares buyer_ref

buyer_ref is absent from create-media-buy-response.json arm 0 (it exists only on the request). Removed the field.

Tests

Added to tests/test_code_generation.py:

• test_sync_creatives_response_creative_covers_all_spec_fields — asserts all 12 schema property names are in Creative.model_fields
• test_create_media_buy_response_arm0_omits_buyer_ref — asserts buyer_ref not in CreateMediaBuyResponse1.model_fields

Verification

make lint, make typecheck, validate-generated, and tests/test_code_generation.py all pass. Regenerated locally from the cached rc.9 schemas (no re-download, no version bump). Variant-renumber churn in other generated files was discarded per CLAUDE.md.

Closes #918
Closes #914

🤖 Generated with Claude Code

[bokelley]

Superseded by #922, which is now merged. #922 fixes this at the root cause — it removes the hand-authored _write_if_needed response-arm snippets in post_generate_fixes.py entirely and schema-derives the arms instead, so the full 12-field Creative (#918) and the buyer_ref removal (#914) come straight from the rc.9 schemas with more faithful types (AnyUrl, pattern-constrained assignment_errors, no | str escape hatch). Closing in favor of #922.