Description

This PR fixes incorrect behavior when editing a Draft Issue with only --title or only --body:

1. gh project item-edit --project-id 1 --id ID_**** --title "new title"
   Previously this updated the title but set the body to an empty string.
   Now the existing body is preserved and only the title is updated.

2. gh project item-edit --project-id 1 --id ID_**** --body "new body"
   Previously this caused a GraphQL error: "Title can't be empty".
   Now the existing title is preserved and only the body is updated.

The GitHub API’s updateProjectV2DraftIssue mutation requires both title and body in the input. When the user passes only one flag, we now fetch the current draft issue, keep the other field unchanged, and send a partial update.

Fixes #12726

Changes

• Flag tracking: Introduced titleChanged and bodyChanged (using cmd.Flags().Changed(...)) so we distinguish “user passed --title” from “user passed --body” and from “user passed both”. This allows correct partial updates and avoids overwriting the other field with an empty string.
• Draft issue fetch: When only one of --title or --body is passed, we fetch the current draft issue via a node(id) query and use its title/body in mutation for the field that wasn’t available.
• queries package: Exposed Query on the shared client so item-edit can run the draft-issue fetch query.
• Tests: Added tests according to the changes