chore: add coverage.out to .gitignore to stop recurring accidental commits

A generated Go test-coverage artifact, `coverage.out`, has been accidentally committed to `master` more than once:
- Re-added with +26,734 lines by PR #12519 (merged 2026-07-08).
- Open PR #12521 is currently trying to remove a prior copy of the same file (-31,030 lines, most of the PR's diff size).

Since this keeps recurring, recommend adding `coverage.out` (and ideally `*.out`) to the root `.gitignore` so it can't be committed again, rather than relying on manual cleanup PRs each time.

This is a root-level file change (`.gitignore`), so per review-bot policy it's filed as an issue rather than auto-fixed directly.

Start with the root .gitignore and review its existing ignore patterns. Add coverage.out, or the proposed *.out rule, then verify that a generated coverage.out is ignored so it cannot be accidentally committed again.

Written by the indexing model from the issue text.