feat(dashboard): trigger key validation on Enter in AddApiKeyModal (#10995)

[rqzbeh]

Fixes #10995 by attaching an onKeyDown Enter handler to the API Key credential input in AddApiKeyModal, allowing users to validate keys by hitting Enter.

[diegosouzapw]

Nice, small UX win — Enter-to-validate is the expected form behavior and wiring into the existing handleValidate()/validating/saving state is exactly right. I ran the new test (passes) and the existing modal component tests against it (all green), so this is safe to merge. Two optional polish notes, not blockers: (1) run npx prettier --write on the new test — the final assert.ok(...) wraps when it fits on one line; and (2) the Enter handler only guards on !validating && !saving, while the Check button also disables on empty key / Google-PSE-missing-cx (AddApiKeyModal.tsx:774). Mirroring that last condition would avoid a pointless validation request when the field is empty. Since this is a UI-only change, the source-string test is an acceptable guard, but a small React render test (like the other add-api-key-modal-*.test.tsx) simulating a keydown Enter would be more robust against future refactors. Great work.