Bring back the commit title and author for Guest users

What does this MR do and why?

Fixes Pipelines list: Guest role sees "Can't find HEA... (#627161 - closed) .

Guests on private projects are seeing "Can't find HEAD commit for this branch" on every row of Build > Pipelines, and no commit title in the pipeline details header. Both read Pipeline.commit, and the Commit type needs read_code, which Guests don't have there. The backend half, Add commit summary fields to the Pipeline Graph... (!255649 - merged), adds commitTitle, commitAuthorName, and commitAuthorGravatar on Pipeline, gated by read_pipeline only. This MR reads them and falls back to them when commit is null.

The list fragment and the header query request the new fields with @gl_introduced(version: "19.5.0"), so a backend that doesn't have them yet strips the fields and returns null instead of an undefinedField error.

commitAuthor in pipeline_url.vue now uses the commit's author name and Gravatar when there is no GitLab user, which covers the unreadable-commit case, and only sets a mailto: link when the REST payload has an email.

Reporters and above see no change. The commit link still 404s for Guests because the commit page needs read_code; that's parity with the REST-era list, not a regression. The short SHA chip for Guests is the separate Draft: Fall back to the pipeline SHA when the c... (!255630 - closed).

Query complexity

The overrides for the three list queries in all_queries_spec.rb move to the measured values: get_pipelines 315 to 317, get_commit_pipelines 271 to 285, get_merge_request_pipelines 628 to 670. The numbers come from the static analyzer, not from an extra Gitaly round trip. The actual Gitaly cost is unchanged: the three fields resolve through the same batched Commit.lazy lookup as commit, one call per page.

Screenshots or screen recordings

	Before	After
Guest, Build > Pipelines
Guest, pipeline header

How to set up and validate locally

1. Create a private project with a repository and at least one pipeline. Keep Settings > CI/CD > General pipelines > Project-based pipeline visibility enabled.
2. Add a user as Guest and sign in as that user.
3. Open Build > Pipelines. On the target branch every row says "Can't find HEAD commit for this branch". On this branch each row shows the commit title link and the author's Gravatar.
4. Open any pipeline. The header's "For commit" line shows the commit title.
5. Repeat as a Reporter: unchanged, the commit popover and GitLab-user avatar still render.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.