Fix Lz4Decoder to propagate UnexpectedEof error on truncated streams

The LZ4 decoder was silently accepting truncated streams by not
validating stream completion in finish().

This issue was discovered by the generic truncated stream test.

Added state tracking to Lz4Decoder:
- Added stream_ended field to track if remaining == 0 was seen
- Modified decode() to set stream_ended = true when stream completes
- Updated finish() to check stream_ended and return UnexpectedEof if false

This matches the behavior of other decoders (bzip2, gzip, etc.) and
ensures applications cannot accidentally accept corrupted or incomplete
LZ4 data as valid.

The generic truncated test now passes for LZ4.

Author: claude

crates/compression-codecs/src/lz4/decoder.rs

@@ -17,6 +17,7 @@ struct DecoderContext {
 #[derive(Debug)]
 pub struct Lz4Decoder {
     ctx: Unshared<DecoderContext>,
+    stream_ended: bool,
 }
 
 impl DecoderContext {
@@ -37,6 +38,7 @@ impl Default for Lz4Decoder {
     fn default() -> Self {
         Self {
             ctx: Unshared::new(DecoderContext::new().unwrap()),
+            stream_ended: false,
         }
     }
 }
@@ -50,6 +52,7 @@ impl Lz4Decoder {
 impl DecodeV2 for Lz4Decoder {
     fn reinit(&mut self) -> Result<()> {
         unsafe { LZ4F_resetDecompressionContext(self.ctx.get_mut().ctx) };
+        self.stream_ended = false;
         Ok(())
     }
 
@@ -74,7 +77,12 @@ impl DecodeV2 for Lz4Decoder {
         };
         input.advance(input_size);
         output.advance(output_size);
-        Ok(remaining == 0)
+
+        let finished = remaining == 0;
+        if finished {
+            self.stream_ended = true;
+        }
+        Ok(finished)
     }
 
     fn flush(&mut self, output: &mut WriteBuffer<'_>) -> Result<bool> {
@@ -92,6 +100,15 @@ impl DecodeV2 for Lz4Decoder {
     }
 
     fn finish(&mut self, output: &mut WriteBuffer<'_>) -> Result<bool> {
-        self.flush(output)
+        self.flush(output)?;
+
+        if self.stream_ended {
+            Ok(true)
+        } else {
+            Err(std::io::Error::new(
+                std::io::ErrorKind::UnexpectedEof,
+                "lz4 stream did not finish",
+            ))
+        }
     }
 }