feat: add Check Runs query type

[pkucia]

Summary

Adds a Check Runs query type that lists the check runs reported against a git reference, backed by
GET /repos/{owner}/{repo}/commits/{ref}/check-runs.

Check runs are the individual checks that GitHub Apps report against a commit: GitHub Actions jobs, external CI,
linters. There is currently no way to see them in Grafana. Workflow Runs covers GitHub Actions at the workflow
level, but not the per-check results that show up on a commit or PR, and not checks reported by other apps.

This makes it possible to build CI health dashboards per commit or branch, such as pass/fail rates by check name and
check latency over time.

Options are Owner, Repository and Ref (required), plus optional Check name, Status and Filter. Filtering is delegated
to the GitHub API rather than done client side, and empty options are omitted from the request so GitHub's own
defaults apply. Full option and response documentation is in docs/sources/query-editor.md.

May partially help #326, since check runs map roughly to jobs and the frame includes a duration per check run.

Implementation

Backend (Go):

• pkg/models/checkruns.go — CheckRunsOptions, with the usual WithRepo variant
• pkg/github/checkruns.go — paginated fetch and frame construction, following Code Scanning for the client
  pattern and Commit Files for the ref scoped shape
• pkg/github/checkruns_handler.go, datasource.go, query_handler.go — handler wired into the query mux
• pkg/github/client/client.go — ListCheckRunsForRef added to the REST client and the Client interface

Uses Checks.ListCheckRunsForRef from go-github, which is already a direct dependency, so there are no new
dependencies.

Frontend (TypeScript/React):

• src/views/QueryEditorCheckRuns.tsx — editor for ref, check name, status and filter
• src/types/query.ts, src/constants.ts, src/validation.ts — CheckRunsOptions type and registration

Docs:

• docs/sources/query-editor.md — list entry plus a ### Check runs reference section

One thing worth flagging: I did not register this in schema.go. The other ref scoped query types (Commit Files,
Pull Request Files, Branches) are not registered either, since there is no table parameter for a free text git
ref, so I followed that precedent. Happy to add it if you would prefer.

completed_at and duration_seconds are nullable so that in flight check runs are represented honestly rather than
with zero values. duration_seconds is derived rather than a raw API field; I can drop it if you would rather keep
the frame a straight mirror of the API.

The editor uses Combobox rather than the deprecated Select, in line with #324.

Test plan

• go test ./...
• yarn test:ci, yarn typecheck, yarn lint, yarn spellcheck, yarn build
• Unit tests for frame construction, pagination, the empty input guard, and filter passthrough, plus a golden
  file for the check_runs frame
• Client tests driving the real go-github client against a mock server, asserting request path, query
  parameters and response parsing
• Frontend test for the new query editor
• Verified against a live Grafana 12.2.0 instance querying real check runs from this repository, covering commit
  SHA refs, heads/<branch> refs, and the check name, status and filter options

Check Suites and the legacy Combined Status endpoint are deliberately out of scope to keep this reviewable. Happy to
follow up with those separately if there is interest.

[cla-assistant]

All committers have signed the CLA.