feat(types): export ProductObjectSchema and extractObjectSchema to restore .extend() on intersection schemas

[bokelley]

Closes #1970

Summary

ProductSchema changed from ZodObject to ZodIntersection in 8.1.0-beta.* to accommodate V1/V2 format variants, silently breaking any downstream that called .extend(), .omit(), or .pick() on it directly. This PR restores the ergonomics via two additive, non-breaking exports and fixes two broken examples in the docs.

New exports:

• ProductObjectSchema — concrete ZodObject for the most common case (ProductSchema.extend(...) → ProductObjectSchema.extend(...)). Directly replaces the documented pattern.
• extractObjectSchema(schema) — generic helper that extracts the right-side ZodObject from any ZodIntersection<L, ZodObject<R>> produced by the codegen, for the other 96 intersection-shaped schemas in 8.1+.

Docs fixed:

• docs/ZOD-SCHEMAS.md — corrected the broken ProductSchema.extend(...) example in the Advanced Usage section.
• docs/ZOD-SCHEMAS.md — corrected the broken ProductSchema.passthrough() example in the Troubleshooting section (.passthrough() is also ZodObject-only).

Both exports include JSDoc examples and are available at import { ProductObjectSchema, extractObjectSchema } from '@adcp/sdk'.

What was tested

• tsc --project tsconfig.lib.json --noEmit — clean (pre-existing deprecation warnings only, unrelated to this change)
• prettier --check — clean on all changed files
• Canary tests added to test/lib/zod-schemas.test.js verifying:
  • ProductObjectSchema has .extend, .omit, .pick
  • ProductObjectSchema.extend({ _test_field: z.string() }).safeParse(...) succeeds on a valid product
  • extractObjectSchema(ProductSchema) returns a ZodObject with .extend
  • The canary explicitly asserts ProductSchema._def.right exists, so a future codegen shape change surfaces as a readable test failure rather than a silent runtime error
• Minor changeset included

Pre-PR review

• code-reviewer: approved — export chain verified intact (schema-helpers.ts → named export in types/index.ts → export * from './types' in index.ts); runtime instanceof guard added per review feedback; canary test strengthened with _def.right pre-assertion.
• dx-expert: approved — naming follows *Schema conventions; doc examples corrected including the passthrough() case that was equally broken; TS2339 → ProductObjectSchema mapping added.

Notes (not blocking)

• A migration-8.0-to-8.1.md doc doesn't exist yet. Developers upgrading from 8.0 will find the fix via the corrected ZOD-SCHEMAS.md example, but a migration doc would be a useful follow-up.
• extractObjectSchema does not handle chained intersections (.and().and()) — TypeScript's type system will surface this at compile time since the argument type won't match ZodIntersection<L, ZodObject<R>>.

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_01A2Thvmn8bFTS5gvJixQS2P

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