feat(types): add Zod schemas for format asset slot shapes

[bokelley]

Refs #1652

Adds FormatAssetSlotSchema, IndividualAssetSlotSchema, RepeatableGroupSlotSchema, all 14 per-type individual slot schemas (IndividualImageAssetSlotSchema, IndividualVideoAssetSlotSchema, …), and 12 group slot schemas (GroupImageAssetSlotSchema, …) as hand-authored Zod companions to the existing TS types in format-asset-slots.ts. The Zod codegen pipeline (generate-zod-from-ts.ts) cannot process the GroupSlotOf<T> mapped type, so these are maintained manually — referencing the existing *AssetRequirementsSchema and OverlaySchema from schemas.generated.ts. Consumers can now validate Format.assets[] from listCreativeFormats() at runtime without maintaining their own Zod union.

This PR ships Part A only. Part B (tightening Format.assets from the incorrect (IndividualImageAsset | … | RepeatableGroupAsset)[] creative-instance types to FormatAssetSlot[]) is a breaking TypeScript type change — needs a major semver bump decision from @bokelley. See #1652 for details.

Note on type/schema divergence: GroupAssetSlot (the TS union in format-asset-slots.ts) still includes GroupBriefAssetSlot and GroupCatalogAssetSlot, and the builder helpers briefGroupAsset/catalogGroupAsset remain. GroupAssetSlotSchema correctly excludes brief/catalog (matching RepeatableGroupAssetSchema in schemas.generated.ts). Aligning the TS types and removing the builders is a breaking change — same semver decision required as Part B.

What was tested

• npm run format:check — passes
• tsc --project tsconfig.lib.json — only pre-existing env errors (missing @types/node, deprecated moduleResolution=node10; confirmed same on main). No new type errors.
• Build skipped: npm run sync-schemas required to populate schemas/cache/ before build:lib can run; this is an environment limitation, not introduced by this PR.

Pre-PR review

• code-reviewer: approved — both blockers (missing rejection tests, GroupBrief/GroupCatalog in union) resolved; nit on min_count/max_count being z.number() vs .int() (consistent with generated schemas)
• ad-tech-protocol-expert: approved at Zod layer — 12-member GroupAssetSlotSchema correctly matches spec and RepeatableGroupAssetSchema; pre-existing TS type divergence (GroupBriefAssetSlot/GroupCatalogAssetSlot in GroupAssetSlot) surfaced as follow-up breaking change

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See adcp#3121
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Session: https://claude.ai/code/session_01RMZexuDcTbgM96xZQwLSjL

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[bokelley]

Superseded by #1654 (merged), which produces the same slot schemas (IndividualAssetSlotSchema, FormatAssetSlotSchema, per-type IndividualImageAssetSchema { asset_type, requirements }, etc.) via codegen rather than hand authoring. Routes the discriminator + requirements through the existing ts-to-zod pipeline so they stay in sync with the spec automatically. Closing.