ci(triage): request the rotation reviewer on pull requests

[DaleSeo]

Motivation and Context

The triage workflow calls issues.addAssignees for both issues and pull requests. For a PR, this only sets the assignee, so the person on rotation never sees it in their "Review requests" queue. This PR updates the workflow to request the on-rotation maintainer as a reviewer on pull requests. Issues still get an assignee, as before.

This PR also fixes three smaller issues with the PR workflow:

• The comment on every PR said "Automated classification did not run," which sounded like something had failed. PRs intentionally skip Codex because auto-label-pr.yml labels them based on the diff.
• GitHub rejects review requests for a PR's author. If the person on rotation opened the PR, the workflow requests a review from the next person on the roster instead.
• When a manual workflow_dispatch run targeted a PR number, the workflow treated it as an issue because it used the event type to determine what kind of item it was. It now checks the item itself.

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[github-actions]

@bolinfest is on triage rotation this week and is the reviewer for this one.

Automated classification did not run for this one, so it needs a full manual pass.

_{Automated triage. Labels are a first pass and the assigned maintainer has the final say.}