feat(compat): v2.5 list_creative_formats + preview_creative adapters; deprecate spec_compat_hooks (stage 5)

[bokelley]

Stage 5 of the versioned-schema-validation port. Follow-up to #658/#659/#664/#665.

What's in scope

Two more v2.5 adapters + the spec_compat_hooks deprecation warning.

New adapters

• list_creative_formats — request pass-through. Response rewrites v2.5 top-level width/height/dimensions into the v3 renders: [{render_id, role, dimensions}] array. Handles template formats (no dims), bare-array responses, and v3-already-shape (idempotent).
• preview_creative — request pass-through. Response renames v2.5 output_id/output_role to v3 render_id/role on every preview render. v3 names win when both present (half-migrated servers don't double-translate). Single-response and batch-response shapes both handled.

Direct ports of src/lib/utils/format-renders.ts and src/lib/utils/preview-normalizer.ts in the JS SDK.

spec_compat_hooks() deprecation

Calling spec_compat_hooks() now emits a DeprecationWarning pointing at adcp.compat.legacy. The implementation moved into a private _spec_compat_hooks_impl so the existing test suite can exercise the legacy behaviour without poisoning pytest's warning filters. Removal target: 6.0.

Stage 5b will

Port the remaining three v2.5 adapters:

• get_products — relies on pricing-adapter.ts (~400 lines in JS, handles fixed_price↔rate, floor_price↔price_guidance.floor, is_fixed discriminator)
• create_media_buy and update_media_buy — share creative-adapter.ts (~300 lines in JS, handles creative_ids↔creative_assignments, null-array coercion, proposal-mode lossy handling)

Each deserves its own PR for reviewer audit.

Stage 4b will

Bundle the real v2.5 JSON schemas (pinned GitHub SHA) so the dispatcher can validate buyer input against v2.5 before the adapter runs. Today the dispatcher only validates the adapter output (against v3).

Test plan

• pytest tests/ -q --ignore=tests/integration — 4475 passed
• 22 new tests across three new files
• ruff check + mypy src/
• DeprecationWarning fires exactly once per spec_compat_hooks() call
• Existing tests at test_spec_compat_hooks.py migrated to use _spec_compat_hooks_impl (no warning noise)

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