Tool output validation fails when outputSchema is a Zod schema other than ZodObject

Describe the bug

Tool output validation fails with the following error when the tool's outputSchema is an optional, nullable, nullish, or union schema (or, most likely anything besides a plain object schema).

Cannot read properties of undefined (reading '_zod')

When validating output the SDK attempts to use normalizeObjectSchema, which returns undefined for anything besides object schemas.

To Reproduce
Steps to reproduce the behavior:

1. Define an outputSchema with something other than a plain object or z.object schema.
2. Implement the tool so that it returns structuredContent that conforms to this schema.
3. Invoke the tool.

// Works:
outputSchema: z.object({ data: z.string() })

// Fails:
outputSchema: z.object({ data: z.string() }).optional()
outputSchema: z.object({ data: z.string() }).nullable()
outputSchema: z.object({ data: z.string() }).nullish()
outputSchema: z.union([z.object({ data: z.string() }), z.object({ value: z.string() })])

Expected behavior
The SDK should be able to validate output based on Zod schemas which represent a JSON object, even if it is optional or a union of possible object schemas. Based on the spec, null is not a valid value for structuredContent, so nullable and nullish schemas should not be allowed, either through more restrictive typing of outputSchema or runtime validation with a clear error message.

Logs

    at isZ4Schema ($PROJECT/node_modules/@modelcontextprotocol/sdk/dist/cjs/server/zod-compat.js:46:21)
    at safeParseAsync ($PROJECT/node_modules/@modelcontextprotocol/sdk/dist/cjs/server/zod-compat.js:80:9)
    at McpServer.validateToolOutput ($PROJECT/node_modules/@modelcontextprotocol/sdk/dist/cjs/server/mcp.js:205:70)
    at $PROJECT/node_modules/@modelcontextprotocol/sdk/dist/cjs/server/mcp.js:135:28
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async wrappedHandler ($PROJECT/node_modules/@modelcontextprotocol/sdk/dist/cjs/server/index.js:131:32)

Additional context

"@modelcontextprotocol/sdk": "1.24.3",
"zod": "4.1.13"

I was expecting this to work based on https://github.com/modelcontextprotocol/typescript-sdk/pull/816 but it has possibly regressed with https://github.com/modelcontextprotocol/typescript-sdk/pull/1040

Start in src/server/mcp.ts at validateToolOutput and read normalizeObjectSchema in src/server/zod-compat.ts, then reproduce the listed optional, nullable, nullish, and union schemas. Done means supported object-shaped schemas validate without the _zod error, while nullable or nullish structuredContent is rejected with a clear error consistent with the specification.