[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

```json
{
"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

```json
{
"operation": "schema"
}
```

### Example query request

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

### Example mutation request

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