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

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": "query_graphql",
  "description": "Execute read-only GraphQL queries against the Data API builder. Supports joins across related entities, filtering, ordering, pagination (first/after), primary-key lookups, and aggregation/groupBy. Use operation 'schema' first to discover entity types, relationships, and available query fields. Then use operation 'query' with a GraphQL query document. NOTE: This tool is read-only; mutations are not supported.",
  "input_schema": {
    "type": "object",
    "properties": {
      "operation": {
        "type": "string",
        "enum": ["query", "schema"],
        "description": "Use 'query' to execute a read-only GraphQL query. Use 'schema' to retrieve available types, relationships, and query fields."
      },
      "query": {
        "type": "string",
        "description": "The GraphQL query document. Required when operation is 'query'. Must be a query operation; mutations are rejected."
      },
      "variables": {
        "type": "object",
        "description": "Optional variables for parameterized GraphQL queries."
      },
      "operationName": {
        "type": "string",
        "description": "Name of the operation to execute when the query document contains multiple named operations."
      }
    },
    "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 } }"
}