test: replace the force_flush test that passes without the override with a reasoned pragma

[lesnik512]

Follow-up to #261, which covered PyroscopeSpanProcessor.force_flush with test_pyroscope_span_processor_does_not_fail_provider_force_flush.

That test cannot fail on the SDK CI installs. The override exists for the declared floor, opentelemetry-sdk>=1.28, whose base SpanProcessor.force_flush has no return and so yields None, making TracerProvider.force_flush() report False. From some later release the base returns True, so with the override deleted the test still passes. Checked directly:

SDK	base force_flush body ends in	TracerProvider.force_flush() with a bare processor
1.28.0	docstring only (returns None)	False
1.45.0	return True	True

pytest resolves at the newest versions and the floors job runs a smoke script, not the suite, so no job runs this test where it could fail. A test that only moves the coverage number is worse than a pragma that says why the line is not exercised, per TS6.

• The test goes.
• return True carries # pragma: no cover - matters only at the sdk 1.28 floor, whose base returns None.

just lint-ci passes; just test-ci: 331 passed, 100 % coverage.