fix: correct json in introspection query results

[mattjohnsonpint]

Description

Fixes HYP-3154 to address #797

Checklist

All PRs should check the following boxes:

• I have given this PR a title using the
  Conventional Commits syntax, leading with fix:,
  feat:, chore:, ci:, etc.
  • The title should also be used for the commit message when the PR is squashed and merged.
• I have formatted and linted my code with Trunk, per the instructions in
  the contributing guide.

If the PR includes a code change, then also check the following boxes. (If not, then delete the
next section.)

• I have added an entry to the CHANGELOG.md file.
  • Add to the "UNRELEASED" section at the top of the file, creating one if it doesn't yet exist.
  • Be sure to include the link to this PR, and please sort the section numerically by PR number.
• I have manually tested the new or modified code, and it appears to behave correctly.
• I have added or updated unit tests where appropriate, if applicable.

[linear]

HYP-3154 GraphQL Introspection Incorrect JSON return

[Copilot]

Pull Request Overview

This PR fixes the JSON output in introspection query results for GraphQL endpoints and updates the changelog accordingly.

• Corrects the condition to handle JSON null values in the GraphQL response.
• Updates the changelog entry to reflect the fix.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
CHANGELOG.md	Added a new changelog entry for the JSON introspection fix
runtime/graphql/graphql.go	Modified condition for handling a null "fields" value in the introspection query results

Comments suppressed due to low confidence (1)

runtime/graphql/graphql.go:172

• [nitpick] Consider renaming the variable 'f' to a more descriptive name (e.g., 'fields') to improve code clarity.

if f := q.Get("fields"); f.Exists() && f.Type == gjson.Null {