fix(github_graphql): set the merged flag on pull requests

[zyv4yk]

⚠️ Pre Checklist

• I have read through the Contributing Documentation.
• I have added relevant tests.
• I have added relevant documentation.
• I will add labels to the PR, such as pr-type/bug-fix, pr-type/feature-development, etc.

Summary

_tool_github_pull_requests.merged is false for every pull request collected through GraphQL, merged ones included.

convertGithubPullRequest in backend/plugins/github_graphql/tasks/pr_extractor.go fills MergedAt, MergedBy and MergeCommitSha, but never Merged, so the column keeps its zero value:

ClosedAt:        pull.ClosedAt,
MergedAt:        pull.MergedAt,
Body:            pull.Body,

#8573 closed the same gap on the REST side — it added Merged: pull.Merged to plugins/github/tasks/pr_extractor.go — but the GraphQL extractor was not part of that change, so a connection running on GraphQL still stores the flag as false.

The flag is derived from mergedAt rather than collected as a separate merged field, for a practical reason: mergedAt is already present in _raw_github_graphql_prs for existing installs, so re-running the extractor repairs rows that are already collected. Querying merged would only fix data going forward and would need a full re-collect for the rest.

The two are equivalent by construction — GitHub sets mergedAt exactly when a pull request is merged. Checked against the API over the 50 most recent pull requests in grafana/grafana (18 merged): zero rows where bool(merged) != (mergedAt != null).

gh api graphql -f query='{ repository(owner:"grafana",name:"grafana"){
  pullRequests(first:50, states:[MERGED,CLOSED,OPEN], orderBy:{field:CREATED_AT,direction:DESC}){
    nodes { number state merged mergedAt } } } }'

Does this close any open issues?

No open issue. It is the GraphQL half of #8571, which was closed by #8573 for the REST collector only.

Other Information

Impact is limited today because the domain layer does not read this column for status: pr_convertor.go computes Status from pr.State == "MERGED" || (pr.State == "closed" && (pr.Merged || pr.MergedAt != nil)), and the GraphQL state is already MERGED. Any consumer that reads _tool_github_pull_requests.merged directly, or any future query that keys on merged = true, gets nothing for GraphQL-collected data.

go build, go vet and golangci-lint run ./plugins/github_graphql/... are clean. pr_extractor_test.go covers merged, closed-unmerged and open. The e2e package needs E2E_DB_URL and was not run locally; the only github_graphql e2e fixture covers deployments, which this PR does not touch.

I do not have permission to set labels; this is pr-type/bug-fix.

[zyv4yk]

Closing this one — the domain layer does not read this column today (pr_convertor.go derives status from the state and mergedAt), so the practical impact is limited, and I would rather not split review attention away from #9086. Happy to reopen if a maintainer wants the flag stored for parity with the REST extractor.