Fix BzDecoder to propagate UnexpectedEof error on truncated streams

Fixes #411

The async BzDecoder was silently accepting truncated bzip2 streams,
returning Ok(0) instead of raising an error. This contrasts with the
synchronous bzip2::read::BzDecoder which properly returns an
UnexpectedEof error.

Added state tracking to BzDecoder:
- Added stream_ended field to track if Status::StreamEnd was received
- Modified decode() to set stream_ended = true on Status::StreamEnd
- Updated finish() to check stream_ended and return UnexpectedEof if false

This ensures applications cannot accidentally accept corrupted or
incomplete compressed data as valid, matching the behavior of the
synchronous decoder.

The generic truncated test now passes for bzip2.

Author: claude

crates/compression-codecs/src/bzip2/decoder.rs

@@ -5,6 +5,7 @@ use std::{fmt, io};
 
 pub struct BzDecoder {
     decompress: Decompress,
+    stream_ended: bool,
 }
 
 impl fmt::Debug for BzDecoder {
@@ -22,6 +23,7 @@ impl Default for BzDecoder {
     fn default() -> Self {
         Self {
             decompress: Decompress::new(false),
+            stream_ended: false,
         }
     }
 }
@@ -49,13 +51,19 @@ impl BzDecoder {
         input.advance((self.decompress.total_in() - prior_in) as usize);
         output.advance((self.decompress.total_out() - prior_out) as usize);
 
+        // Track when stream has properly ended
+        if status == Status::StreamEnd {
+            self.stream_ended = true;
+        }
+
         Ok(status)
     }
 }
 
 impl DecodeV2 for BzDecoder {
     fn reinit(&mut self) -> io::Result<()> {
         self.decompress = Decompress::new(false);
+        self.stream_ended = false;
         Ok(())
     }
 
@@ -101,6 +109,13 @@ impl DecodeV2 for BzDecoder {
     }
 
     fn finish(&mut self, _output: &mut WriteBuffer<'_>) -> io::Result<bool> {
-        Ok(true)
+        if self.stream_ended {
+            Ok(true)
+        } else {
+            Err(io::Error::new(
+                io::ErrorKind::UnexpectedEof,
+                "bzip2 stream did not finish",
+            ))
+        }
     }
 }