fix(schemas): import from zod/v4 to match SDK's zod API (#548)

The generated schemas compose with schemas imported from
@modelcontextprotocol/sdk/types.js (e.g. z.array(ContentBlockSchema)).
The SDK constructs its schemas via `import * as z from 'zod/v4'`, but
our generated schema.ts used `import { z } from "zod"` — which returns
the v3 API when the consumer has zod 3.25.x installed.

Mixing v3-created and v4-created schema instances throws at parse time
with errors like `keyValidator._parse is not a function` because zod v4
dropped the `_parse` / `_parseSync` internals that v3 wrappers call.

This was masked before #534 because zod was bundled — all schema
creation shared the same inlined zod copy. Externalizing zod exposed
the mismatch on consumers pinned to zod 3.x (e.g. anthropics/apps).

The `zod/v4` subpath is exported by both zod 3.25+ and zod 4.x, so the
existing peerDependency range `^3.25.0 || ^4.0.0` is preserved.

https://claude.ai/code/session_013xQUegrvVGct5xpqeSL5av

Co-authored-by: Claude <noreply@anthropic.com>

src/generated/schema.test.ts

@@ -1,7 +1,7 @@
 // Generated by ts-to-zod
 // Integration tests verifying schemas match TypeScript types
 // Run: npm run generate:schemas
-import { z } from "zod";
+import { z } from "zod/v4";
 
 import * as spec from "../spec.types.js";
 import * as generated from "./schema.js";

src/generated/schema.ts

@@ -1,7 +1,7 @@
 // Generated by ts-to-zod
 // Post-processed for Zod v3/v4 compatibility and MCP SDK integration
 // Run: npm run generate:schemas
-import { z } from "zod";
+import { z } from "zod/v4";
 import {
   ContentBlockSchema,
   CallToolResultSchema,