fix(testserver): report a silenced output, and rename the never-rendered status

[schloerke]

Fixes both test_server() status issues; they are coupled, since the second frees the name the first needs.

#2492 — a silent req() failure is invisible

set_silent() left the test-mode record untouched, so get_output() kept reporting the previous render's value with status="ok" while the browser blanked the output. A test could pass while asserting a value the user cannot see.

OutBoundMessageQueues now records the suppression as its own fact in test_silent, discarded as soon as the output computes a value or errors again. test_values/test_errors still retain the last computed value (Shiny for R parity), so _build_test_snapshot() and the HTTP snapshot are unchanged — only test_server reads the new set, the same way it already reads test_tracebacks.

ts.set_inputs(n=1)
assert ts.get_output("txt") == "ok: 1"
ts.set_inputs(n=-1)                                # fails req()
assert ts.get_output("txt").status == "silent"     # now true
assert ts.get_output("txt").value is MISSING       # no stale value

#2493 — "silent" meant "never rendered"

With silence observable, "silent" now means what Shiny already taught: this render produced nothing. The state it used to name — an output that has not run at all — is "never-rendered".

In practice most outputs that used to report "silent" still do, because reading an unset input raises a SilentException. "never-rendered" is the narrower state of an output whose effect never ran, e.g. a hidden (suspended) one.

__eq__ now has a distinct message per state; keys()/dict()/repr() treat both the same as before (no value, no error).

No changelog entry: test_server() is itself unreleased (#2470), so its documented behavior is just what ships.

Verification

.venv/bin/python -m pytest tests/pytest -q → 1208 passed, 1 skipped. uv run make format check-lint check-types and make check-pyright clean. New tests cover the silenced-after-ok transition, the never-rendered state, and the queue-level clearing rules; the bundled shiny-for-python skill's status table is updated.