Parse ALLOW_MUTATIONS as string

[mweidner037]

Before:

$ ALLOW_MUTATIONS=true npx mcp-graphql
...
ZodError: [
  {
    "code": "invalid_type",
    "expected": "boolean",
    "received": "string",
    "path": [
      "ALLOW_MUTATIONS"
    ],
    "message": "Expected boolean, received string"
  }
]

After: Working as expected.

Snippet from colinhacks/zod#2985 (comment)

[blurrah]

Good one, normally I often also allow for 1 and 0 and refine it within zod but this is good enough 😄

Thanks for the fix!