fix(tea-plugin): surface an operator-visible notice on fail-open gate verdicts

[gmoncor]

What: When every configured artifact for a blocking gate returns a None verdict, emit a visible stderr notice naming the gate and that this is being treated as fail-open/pass, instead of silently continuing.

Why: The fail-open behavior itself is a deliberate, documented, and tested design choice and this PR does not change it; but today a fail-open pass leaves zero operator-visible trace.

How:

• Add a stderr print at the point where artifact lookup yields a None verdict for every configured artifact of a gate, reusing the print-to-stderr convention already used by unity_setup.py/unity_teardown.py.
• Do not change GATE_ARTIFACTS, the '|'-skip logic, or the fail-open return value.

Testing: Extend the existing test_missing_artifact_is_fail_open / test_garbled_artifact_is_fail_open tests to also assert on captured stderr, confirming the commit still proceeds.

Changelog: "Added: the TEA plugin now prints an operator-visible notice when a gate's artifacts can't be found or parsed and the gate fails open."

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