Auto-bump PatchVersion on release branch after each release

Follow-up to #15681.

Now that dynamic Helix package-version computation is in place (#15681), bumping the `PatchVersion` in `eng/Versions.props` after a release is a **single-line change** — no need to also hand-edit `Directory.Packages.Helix.props`. This makes it practical to automate the bump as part of the release workflow.

**Proposal:** add a `bump-patch-pr` job to `.github/workflows/release-github-tasks.yml` that, after the release tag/release have been created, opens a PR against the just-released `release_branch` that bumps `` in `eng/Versions.props`. For example, after shipping `13.4.0` from `release/13.4`, the workflow would automatically open a PR on `release/13.4` rolling to `13.4.1`, leaving the branch ready for the next servicing build without manual editing.

**Design notes** (mostly mechanical, following the existing patterns in the workflow):
- Run in parallel with `merge-pr` / `baseline-pr` after `release` completes.
- Gate on `is_prerelease != true` (previews don't roll the patch).
- Sanity-check that the `release_branch`'s `eng/Versions.props` `Major.Minor` matches the just-released version (catches wrong-`release_branch` mistakes).
- Idempotent via a new `skip_bump_patch_pr` input + an "existing PR?" check mirroring the other PR-creation jobs.
- Plumbing: thread `skip_bump_patch_pr` through `eng/pipelines/release-publish-nuget.yml`'s AzDO dispatch hashtable; update `docs/release-process.md`.

**Timing:** ideally land *after* the next 13.3 servicing release (if one happens) so we don't have to backport / coordinate. The dynamic-version mechanism in #15681 only lives on `main` going forward — older release branches like `release/13.3` still have the hardcoded `Directory.Packages.Helix.props` and would break if the auto-bump tried to roll their patch without a matching backport.

A draft implementation exists at commit f7e9e1406 on the joperezr/automate-helix-versions branch (later reverted from #15681 to limit that PR's scope) — can be revived as the starting point when picking this up.