Allow selecting an exact E2E test for PR workflows

### Problem

The PR E2E workflow always runs its baseline suites plus changed E2E files. Maintainers cannot manually target one E2E
file or one exact test against a PR head, which makes focused verification slower and more expensive.

### Proposed behavior

Add manual workflow inputs that:

- Check out a specified PR head.
- Select one tracked `e2e-tests/**/*.test.ts` file.
- Optionally select the exact test declared at a source line or match a Vitest test-name pattern.
- Preserve the existing automatic PR behavior when no selector is provided.
- Validate all user-provided selectors before invoking Vitest.

Start with the PR E2E workflow and its existing baseline and changed-file behavior. Trace how the workflow checks out a PR head and invokes Vitest, then add and validate inputs for one tracked e2e-tests/**/*.test.ts file, an optional source line, or a test-name pattern. Done means selectors are rejected before Vitest and empty inputs preserve automatic PR runs.

Written by the indexing model from the issue text.