build(mocks): Generate mocks with the pinned mockgen and check them in CI

[manjari25]

Why?

make mocks resolved mockgen from PATH, and nothing in the repo pinned or documented which one to use. On a machine with a different mockgen (for example the older github.com/golang/mock binary), it rewrote every checked-in mock with no real changes and failed on generic interfaces such as stovepipe/extension/sourcecontrol. The contributor then had to work out which of the changed files mattered.

CI also never ran make check-mocks, so a PR that changed an interface without regenerating its mock was only caught if a test happened to use the stale mock.

What?

• make mocks builds the mockgen pinned in go.mod through Bazel (@org_uber_go_mock//mockgen), copies it to the git-ignored bin/mockgen/, and puts it first on PATH for the go generate run. No local install is needed, and the //go:generate directives are unchanged.
• The Tidy CI job runs make check-mocks, so stale mocks fail the required checks.
• doc/howto/DEVELOPMENT.md notes both behaviors.

Test Plan

On a machine where the mockgen on PATH is not the pinned one (this was verified on one such machine):

1. Run make mocks and check that it completes and git status is clean.
2. Run make check-mocks, make lint, make check-gazelle, and make check-tidy on the committed branch. All pass.

The new CI step hasn't run on GitHub yet, so this PR's Tidy job is its first run.

Issue