tests: migrate gtest suite to the ffmpeg_bridge API and add bridge coverage

- Replace direct FFmpeg usage in the test suite (channel layout masks,
  pixel/sample format constants, AVFrame field access, sws_scale) with
  the bridge equivalents and the olive::AVFrame adapter
- Drop CoreRational.ToAVRational (API removed with core's FFmpeg
  dependency) and the GetSwsColorspaceFromAVColorSpace tests (helper
  moved inside the bridge)
- New ffmpeg_bridge_test.cpp exercises the C API directly: constants vs
  core, error strings, pixel format utilities, frames/packets, scaler,
  resampler, audio graph tempo processing, probe/decoder round-trip on
  tests/demo.mp4 (including hw-frame transfer), SRT subtitle reading,
  and encoder end-to-end tests (PNG video and PCM audio probed back)

tests/gtest/module_smoke_test.cpp

@@ -49,8 +49,8 @@ TEST(ModuleSmoke, HumanStringsSampleRate)
 TEST(ModuleSmoke, HumanStringsChannelLayout)
 {
 	EXPECT_FALSE(
-		olive::HumanStrings::ChannelLayoutToString(AV_CH_LAYOUT_MONO).isEmpty());
-	EXPECT_FALSE(olive::HumanStrings::ChannelLayoutToString(AV_CH_LAYOUT_STEREO)
+		olive::HumanStrings::ChannelLayoutToString(olive::core::kChannelLayoutMono).isEmpty());
+	EXPECT_FALSE(olive::HumanStrings::ChannelLayoutToString(olive::core::kChannelLayoutStereo)
 					 .isEmpty());
 }