test: replace fake and duplicate tests with real assertions

Fake tests rewritten to assert real behavior:
- audio_smoke: conversion tests now actually Convert() samples and verify
  output; waveform length/summary assertions tightened to exact values
- plugin_format_conversion: RowBytes/U8ToU16/LoadImageFile now call real
  production code (VideoParams::GetBytesPerPixel, sws scaler, OIIO decode
  of tests/img.png with known pixel values)
- core_color: HSV round trip now verifies fromHsv(toHsv(c)) == c instead
  of comparing toHsv against its own accessors
- core_bezier/node_inputimmediate: expected values replaced with
  independently derived constants instead of re-running the code under test
- common_commandlineparser/common_debug/common_jobtime: capture
  stdout/stderr/qDebug and assert actual output content
- proxy_manager: ProxyFinished test now drives a real proxy job instead of
  emitting the signal itself; proxy_dialog/panel/proxy/preferences/timeruler
  tests assert real widget state
- viewer_smoke/preview_autocacher/render_misc: zero-assertion tests given
  observable-state assertions or removed where nothing is observable

Duplicates removed:
- plugin_smoke_test.cpp: 18 tests duplicated from plugin_paraminstance /
  plugin_support_* / plugin_renderer_readback (751 -> 180 lines)
- module_smoke HumanStrings tests covered precisely by ui_humanstrings_test
- render_misc duplicate kDefaultInterpolation constant check

Removed by policy (skip allowed, never disabled):
- all DISABLED_ prefixes: re-enabled as real offscreen tests or deleted
- ffmpeg_decoder_hw: hardcoded personal path replaced with
  OAK_TEST_HW_DECODE_FILE env var, GTEST_SKIP when unset

Also:
- config_test: restore Config defaults after run (cross-test pollution)
- render_worker_footage: drop /tmp debug-output scaffolding
- previewaudiodevice construction test asserts the real bugfix

tests/gtest/common_commandlineparser_test.cpp

@@ -1,7 +1,23 @@
 #include <gtest/gtest.h>
 
+#include <QDebug>
+
 #include "common/commandlineparser.h"
 
+namespace
+{
+
+// Collects qDebug/qWarning/qCritical output for inspection
+QStringList g_captured_messages;
+
+void CaptureMessageHandler(QtMsgType, const QMessageLogContext &,
+						   const QString &msg)
+{
+	g_captured_messages.append(msg);
+}
+
+} // namespace
+
 TEST(CommonCommandLineParser, OptionWithoutArgument)
 {
 	CommandLineParser parser;
@@ -57,16 +73,34 @@ TEST(CommonCommandLineParser, UnknownOptionWarning)
 	CommandLineParser parser;
 	parser.AddOption({ QStringLiteral("known") }, QStringLiteral("Known"));
 
-	// Should not crash; unknown option is logged
+	g_captured_messages.clear();
+	QtMessageHandler old = qInstallMessageHandler(CaptureMessageHandler);
 	parser.Process({ QStringLiteral("app"), QStringLiteral("-unknown") });
+	qInstallMessageHandler(old);
+
+	// The warning must name the offending option
+	ASSERT_EQ(g_captured_messages.size(), 1);
+	EXPECT_TRUE(g_captured_messages.first().contains(
+		QStringLiteral("Unknown parameter:")));
+	EXPECT_TRUE(
+		g_captured_messages.first().contains(QStringLiteral("-unknown")));
 }
 
 TEST(CommonCommandLineParser, UnknownPositionalWarning)
 {
 	CommandLineParser parser;
 
-	// Should not crash; unknown positional is logged
+	g_captured_messages.clear();
+	QtMessageHandler old = qInstallMessageHandler(CaptureMessageHandler);
 	parser.Process({ QStringLiteral("app"), QStringLiteral("extra") });
+	qInstallMessageHandler(old);
+
+	// The warning must name the offending positional argument
+	ASSERT_EQ(g_captured_messages.size(), 1);
+	EXPECT_TRUE(g_captured_messages.first().contains(
+		QStringLiteral("Unknown parameter:")));
+	EXPECT_TRUE(
+		g_captured_messages.first().contains(QStringLiteral("extra")));
 }
 
 TEST(CommonCommandLineParser, HiddenOptionExcludedFromHelp)
@@ -78,6 +112,17 @@ TEST(CommonCommandLineParser, HiddenOptionExcludedFromHelp)
 	parser.AddPositionalArgument(QStringLiteral("file"),
 								 QStringLiteral("Input file"));
 
-	// Should not crash; hidden option should be skipped during help output
+	// PrintHelp writes to stdout via printf
+	testing::internal::CaptureStdout();
 	parser.PrintHelp("/usr/bin/app");
+	std::string help = testing::internal::GetCapturedStdout();
+
+	// Visible option and positional argument must be listed
+	EXPECT_NE(help.find("-visible"), std::string::npos);
+	EXPECT_NE(help.find("Visible"), std::string::npos);
+	EXPECT_NE(help.find("[file]"), std::string::npos);
+
+	// Hidden option must not appear anywhere in the help text
+	EXPECT_EQ(help.find("-hidden"), std::string::npos);
+	EXPECT_EQ(help.find("Hidden"), std::string::npos);
 }