Backport of CSI: enforce single access mode at validation time into release/1.1.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #12337 to be assessed for backporting due to the inclusion of the label backport/1.1.x.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

The below text is copied from the body of the original PR.

---

Fixes #10157

A volume that has single-use access mode is feasibility checked during
scheduling to ensure that only a single reader or writer claim
exists. However, because feasibility checking is done one alloc at a
time before the plan is written, a job that's misconfigured to have
count > 1 that mounts one of these volumes will pass feasibility
checking.

Enforce the check at validation time instead to prevent us from even
trying to evaluation a job that's misconfigured this way.

Rework the validation code for legibility and add missing unit test.

[hashicorp-cla]

Thank you for your submission! We require that all contributors sign our Contributor License Agreement ("CLA") before we can accept the contribution. Read and sign the agreement

Learn more about why HashiCorp requires a CLA and what the CLA includes

---

temp seems not to be a GitHub user.
You need a GitHub account to be able to sign the CLA. If you already have a GitHub account, please add the email address used for this commit to your account.

_{Have you signed the CLA already but the status is still pending? Recheck it.}

[lgfa29]

Manually backported

[github-actions]

I'm going to lock this pull request because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.