Allow selecting an exact E2E test for PR workflows

[jesseturner21]

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.