Upgrades the Zod validation library from version 3 to version 4.3.6.

### Changes

- Updated deno.json to specify zod@4 instead of zod@3
- Updated deno.lock with Zod 4.3.6 dependency resolution

### Compatibility

All existing Zod usage in the codebase is fully compatible with v4:
- Schema definitions (model input/resource schemas)
- Type inference with z.infer
- Validation methods (parse, safeParse)
- Schema composition and chaining

### Testing

- ✅ Type checking passes (deno check)
- ✅ All Zod-related tests pass (59 tests across model domain layer)
- ✅ Integration tests pass
- ✅ 145/146 tests pass (1 unrelated pre-existing failure in logger tests)

### Migration Notes

No code changes required. Zod v4 maintains API compatibility with v3 for the features used in this codebase.