Implement the createDiscussion GraphQL mutation in the discussion client.
- Add getRepositoryMeta helper to resolve repo node ID and check
discussions-enabled flag before mutating
- Skip repo lookup when CreateDiscussionInput.RepositoryID is provided
- Reuse discussionListNode mapping for consistent field coverage
- Table-driven tests: field mapping, pre-resolved repo ID, discussions
disabled, repo not found, mutation error
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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