graphql: add query depth limit to prevent DoS attacks

[grandchildrice]

Summary

This PR addresses a DoS vulnerability in the GraphQL service by implementing a maximum query depth limit. While #26026 introduced timeout handling, it didn't fully mitigate the attack vector where deeply nested queries can still consume excessive CPU and memory resources before the timeout is reached.

Changes

• Added maxQueryDepth constant (set to 20) to limit the maximum nesting depth of GraphQL queries
• Applied the depth limit using graphql.MaxDepth() option when parsing the schema
• Added test case TestGraphQLMaxDepth to verify that queries exceeding the depth limit are properly rejected

Security Impact

Without query depth limits, malicious actors could craft deeply nested queries that:

• Consume excessive CPU cycles during query parsing and execution
• Allocate large amounts of memory for nested result structures
• Potentially cause service degradation or outages even with timeout protection

This fix complements the existing timeout mechanism by preventing resource-intensive queries from being executed in the first place.

Testing

Added TestGraphQLMaxDepth which verifies that queries with nesting depth > 20 are rejected with a MaxDepthExceeded error.

References

• Original issue: Graphql: How to cancel a graphql query for protect DOS attack? #26026
• Related security best practices: https://www.howtographql.com/advanced/4-security/

[fjl]

We'll have to see if this limit is too tight for real world uses, but it's good to add.