fix(build): commit generated mocks so fresh clones work with plain Go tooling

[bujjibabukatta]

Closes: #9088

Summary

backend/mocks/ is gitignored but imported by tracked source code
(helpers/unithelper) and test files (helpers/pluginhelper/api).
On a fresh clone this breaks go mod tidy, go build ./..., IDE
tooling, and Dependabot's gomod ecosystem.

Root Cause

The mocks are generated via mockery but never committed. Since
helpers/unithelper imports them directly (not as test-only code),
Go can't find these packages on a clean checkout and tries to fetch
them as external modules, which fails.

Fix

• Removed backend/mocks/ from .gitignore.
• Committed the generated mock files (make mock, mockery v3.7.4).
  No changes to go.mod/go.sum.
• Added a CI step to run make mock and diff against the committed
  mocks, so they can't silently go stale.