Scope CI_MERGE_REQUEST_APPROVED preload to avoid jobs/request timeout

What does this MR do and why?

POST /api/v4/jobs/request can time out (Rack::Timeout::RequestTimeoutException, ~60s) while resolving the predefined CI_MERGE_REQUEST_APPROVED variable, instead of returning a job payloadn -affected jobs then hang until their timeout with no trace output. See #606211.

An interaction between two independent changes, amplified by MR size:

1. Unbounded preload (2025): the EE approval-variable preloader unconditionally preloads merge_request_diff -> merge_request_diff_commits -> commit_author/committer` over all commits in the MR diff. That data is only read when the project has "Prevent approval by users who add commits" (merge_requests_disable_committers_approval?) enabled , yet it's preloaded regardless of the setting or commit count.
2. Lazy resolution moved to the request path shifting that preload out of pipeline creation and into POST /api/v4/jobs/request, inside the 60s Rack window.

The fix

Behind feature flag ci_optimize_merge_request_approved_resolution (default disabled):

1. Scope the commit preload to only run when merge_requests_disable_committers_approval? is enabled — the only case that reads committer identity. For every other project (the default), the expensive preload no longer fires.
2. git_depth_value uses a targeted variables["GIT_DEPTH"] lookup instead of to_hash, so reading one variable no longer forces resolution of all lazy variables.

Both changes sit behind the same flag so they roll back together.

How to set up and validate locally

1. Create an MR whose diff has a large number of commits. Then run a merge request pipeline.
2. Register a job via POST /api/v4/jobs/request:

• Flag off: resolves CI_MERGE_REQUEST_APPROVED, preloading all diff commits. This will be slow, times out on very large MRs.
• Flag on (project with committer-approval off): preload skipped, request returns promptly.

#606211