Run commands.def validation in an independent CI job (#4753)

Pointed out by @madolson in #4747 

#### Motivation

The existing validation still works, but it runs near the end of the
large `test-ubuntu-latest` job. In both recent cases, the stale
`commands.def` was detected only after the originating change had
merged.

- Path hash was merged in #4506 with a stale `commands.def`. The next
successful CI execution reached the validation step and [failed on
`commands.def`](https://github.com/valkey-io/valkey/actions/runs/35109002068/job/104838099246),
resulting in #4709.
- DELEX was merged in #4607 with a stale `commands.def`. A later,
unrelated PR reached the validation step and [failed on
`commands.def`](https://github.com/valkey-io/valkey/actions/runs/35612624270/job/106375287422),
resulting in #4747.

Therefore, the validation was still functioning, but it did not produce
an isolated result on the originating PR before merge.

This change creates a dedicated `validate-commands-def` job that runs
independently and fails directly on the PR introducing stale generated
output.

Signed-off-by: Sarthak Aggarwal <sarthagg@amazon.com>