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/config_test.cpp

@@ -30,6 +30,14 @@ TEST(Config, SetAndGetValues)
 
 	cfg[QStringLiteral("UnitTestDouble")] = 3.14;
 	EXPECT_NEAR(cfg[QStringLiteral("UnitTestDouble")].toDouble(), 3.14, 0.001);
+
+	// Config offers no key-removal API, so reset the singleton to its
+	// default state to avoid leaking the UnitTest* keys into later tests
+	cfg.SetDefaults();
+	EXPECT_FALSE(cfg[QStringLiteral("UnitTestValue")].isValid());
+	EXPECT_FALSE(cfg[QStringLiteral("UnitTestString")].isValid());
+	EXPECT_FALSE(cfg[QStringLiteral("UnitTestBool")].isValid());
+	EXPECT_FALSE(cfg[QStringLiteral("UnitTestDouble")].isValid());
 }
 
 TEST(Config, MissingKeyReturnsInvalidVariant)