[Enh]: add `query_graphql` DML tool to MCP server

[JerryNixon]

What?

Allow agents to make complex queries that include join operations.

Why?

Join operations allow agents to retrieve related information across entities, enabling richer reasoning and discovery scenarios that single-entity queries cannot provide.

How?

Expose a single GraphQL MCP tool whose behavior is controlled by an enum parameter.

Example tool signature

{
  "name": "graphql",
  "description": "Execute GraphQL queries, mutations, or retrieve the schema.",
  "input_schema": {
    "type": "object",
    "properties": {
      "operation": {
        "type": "string",
        "enum": ["query", "mutation", "schema"],
        "description": "Type of GraphQL operation."
      },
      "query": {
        "type": "string",
        "description": "GraphQL query or mutation text. Not required when operation = schema."
      },
      "variables": {
        "type": "object",
        "description": "Optional GraphQL variables."
      }
    },
    "required": ["operation"]
  }
}

Behavior.

operation = query
Execute the provided GraphQL query.

operation = mutation
Execute the provided GraphQL mutation.

operation = schema
Return the GraphQL schema or a simplified schema description.

Example schema request

{
  "operation": "schema"
}

Example query request

{
  "operation": "query",
  "query": "query { products(first:5) { productId name category { name } } }"
}

Example mutation request

{
  "operation": "mutation",
  "query": "mutation { createProduct(item:{ name:\"Widget\" }) { productId name } }"
}