Give three skipped tests the isolation their skip reason asked for

[JE-Chen]

test_r3_gui_thread_marshal.py carried three @pytest.mark.skips whose own
reason named the fix:

needs subprocess isolation (see test_actions_menu_gui) … skip until then

They cover real wiring — that a file received on a WebRTC worker thread reaches
the GUI thread through a queued signal rather than a thread-affine
QTimer.singleShot, and that the admin console's thumbnail poll deletes its
QThread each tick instead of leaking one per interval.

Skipping them was right at the time. Building the WebRTC panel or the admin
console and then tearing a worker QThread down aborts the shared pytest
process under offscreen Qt. And because deleteLater is a no-op until an event
loop runs, the abort does not land in the test that caused it — it detonates
inside some later, unrelated file, with no traceback.

What changed

One probe runs all three checks in a child process, writes a JSON verdict per
check, and os._exit(0)s without teardown — the same shape
test_actions_menu_gui already uses for the full tab set.

Verdicts are ok, failed: … or unavailable: …, so a machine without the
[webrtc] extra (CI's pytest-headless) reports a skip, while a machine
that has it actually checks the wiring. That distinction matters: a blanket
importorskip would have hidden a real failure on developer machines too.

The checks have teeth — verified, not assumed

Deleting the single line thread.finished.connect(thread.deleteLater) from
admin_console_tab.py:

'thumbnail_reaped': 'failed: the QThread outlived finish'

…while panel_signals and webrtc_marshal stayed green, so the failure is
localised to the wiring it covers.

Result

The headless suite now runs end to end with no --ignore flags — 4,815
passing, exit 0. The 17 remaining skips are all optional-dependency or
platform gates (Darwin-only path, no module named openpyxl, …). No "skip
until then" is left in the suite.

Progress.md loses the entry, per the rule that it records only open items.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 0 complexity · 1 duplication

Metric	Results
Complexity	0
Duplication	1

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