declare module 'zod' augmentation breaks when third-party packages load both 'zod' and 'zod/v4' subpaths #379

The declare module 'zod' augmentation on ZodType breaks when third-party packages cause TypeScript to load both the top-level zod entry point and the zod/v4 subpath. The top-level zod exports a v3-compat ZodType<Output, Def, Input>, while zod/v4 exports the native v4 ZodType<Output, Input, Internals>. TypeScript merges both into a single ZodType with incompatible type parameters, causing .openapi() to disappear from all schemas.

Minimal repro: install zod@^4 + zod-to-openapi@^8 + langchain + better-auth (or any package whose .d.ts loads zod/v4). npx tsc --noEmit fails. Remove any one of the three ingredients and it passes.

Fix submitted in #378.