podman ps: allow sorting by pod (finishes #13033's --sort half)

[ROKUMATE]

SortPsOutput already had a case "pod" backed by the psSortedPod sorter, but pod
was not among the --sort flag's accepted values (validate.Value), so it was rejected
before ever reaching the sorter — the sort option was unreachable. This wires it up by
adding pod to the four surfaces that excluded it: the --sort accepted values, shell
completion, the invalid-option error message, and the man page. Completes the --sort
part of the RFE in #13033 (the --format part already works).

Checklist

Ensure you have completed the following checklist for your pull request to be reviewed:

• I have read and understood our contributing guidelines and will not have more than two open PRs as a new contributor.
• PR description, commit message, and GitHub comments are human-written, per LLM Policy
• Certify you wrote the patch or otherwise have the right to pass it on as an open-source patch by signing all
  commits. (git commit -s). (If needed, use git commit -s --amend). The author email must match
  the sign-off email address. See CONTRIBUTING.md
  for more information.
• Referenced issues using Fixes: #00000 in commit message (if applicable)
• Tests have been added/updated (or no tests are needed)
• Documentation has been updated (or no documentation changes are needed)
• All commits pass make validatepr (format/lint checks)
• Release note entered in the section below (or None if no user-facing changes)

Does this PR introduce a user-facing change?

[Honny1]

Can you check if this is leftover or unfinished #13033 ?

[ROKUMATE]

@Honny1 yeahh checked it ... was now thinking to implement the left part of it ...
will add "pod" to the 4 surfaces that currently exclude it (validate.Value, completion, the error message, the man page)

also will change the pr description from removing to adding a complete feature of --sort=pod from the issue you mentioned

[ROKUMATE]

Updated the pr , title and description accordingly
right now once decision i made as
exposing the existing pod-ID sort; happy to switch to pod-name if preferred 👍🏼

[Honny1]

I think it should be sorted by name. Don't forget the e2e tests.

[ROKUMATE]

@Honny1 Done 👍🏼