Fix empty filtered runs passing

[vsolano9]

Summary\n\n- fail runs when tag filtering selects zero cases, even without a threshold\n- explain empty filtered runs in the terminal report\n- add regression coverage for both behaviors\n\nFixes #6\n\n## Verification\n\n-

RUN v1.6.1 /private/tmp/evalgate-oss-next

✓ tests/runner.test.ts (10 tests) 3ms
✓ tests/reporters.test.ts (7 tests) 2ms

Test Files 2 passed (2)
Tests 17 passed (17)
Start at 16:08:00
Duration 202ms (transform 63ms, setup 0ms, collect 117ms, tests 5ms, environment 0ms, prepare 79ms) (17 passed)\n- regression test observed failing before the runner change and passing after it

[github-actions]

Nice work @vsolano9, this is merged.

Contributions like this are what keep Evalgate moving.

A star on the repo would mean a lot if this project is useful to you.

Happy to see more from you whenever you have the time.