fix(pools): return pooled gzip reader on Reset error

GetGzipReader got a reader from the pool, Reset it, and on error returned a
fresh gzip.NewReader while never returning the pooled reader to the pool —
one leaked reader per reset error (non-gzip input), silently draining the
pool. Return the pooled reader without Close (Close panics because the
decompressor is nil until a header parses); the next caller's Reset
reinitialises it.

Test: TestGetGzipReader_ResetErrorPath (error path leaves the pool usable
for subsequent valid gzip reads).

pkg/pools/buffer.go

@@ -78,7 +78,12 @@ func PutGzipWriter(gz *gzip.Writer) {
 func GetGzipReader(r io.Reader) (*gzip.Reader, error) {
 	gr := gzipReaderPool.Get().(*gzip.Reader)
 	if err := gr.Reset(r); err != nil {
-		// If reset fails, create a new reader
+		// Reset failed (e.g. non-gzip data): return the pooled reader so it
+		// isn't leaked, and hand back a fresh one for this call. We must not
+		// call gr.Close() here — the decompressor is nil until a header
+		// parses, so Close would panic. The next GetGzipReader's Reset
+		// reinitialises it.
+		gzipReaderPool.Put(gr)
 		return gzip.NewReader(r)
 	}
 	return gr, nil

pkg/pools/buffer_test.go

@@ -415,3 +415,31 @@ func BenchmarkWithoutPool(b *testing.B) {
 		}
 	})
 }
+
+// TestGetGzipReader_ResetErrorPath exercises the path where the pooled reader
+// fails to Reset (non-gzip input). The pooled reader must be returned to the
+// pool and a usable reader handed back; a subsequent valid gzip read must
+// still succeed so the pool is not left in a broken state.
+func (suite *BufferPoolTestSuite) TestGetGzipReader_ResetErrorPath() {
+	t := suite.T()
+	// Non-gzip input -> Reset fails -> fresh reader returned (with error).
+	gr1, err := GetGzipReader(strings.NewReader("not gzip data at all"))
+	assert.Error(t, err, "expected error decoding non-gzip data")
+	if gr1 != nil {
+		PutGzipReader(gr1)
+	}
+
+	// Valid gzip input afterwards must still work, proving the pool is usable.
+	var buf bytes.Buffer
+	zw := gzip.NewWriter(&buf)
+	_, werr := zw.Write([]byte("hello world"))
+	assert.NoError(t, werr)
+	assert.NoError(t, zw.Close())
+
+	gr2, err := GetGzipReader(bytes.NewReader(buf.Bytes()))
+	assert.NoError(t, err)
+	defer PutGzipReader(gr2)
+	decoded, rerr := io.ReadAll(gr2)
+	assert.NoError(t, rerr)
+	assert.Equal(t, "hello world", string(decoded))
+}