fix(compression): fail truncated zstd frames instead of spinning

data_decompress_limited() looped while ZSTD_decompressStream() returned a
positive hint.  A truncated frame keeps returning that hint with all input
consumed, so a malformed/truncated payload spun forever (CPU DoS).  Detect
input exhaustion with an incomplete frame and fail via the existing cleanup,
skipping the check when the output buffer merely needs to grow first.

Add a fork+alarm regression test that truncates a valid frame and asserts
decompression returns NULL promptly.

src/shared/compression.c

@@ -324,6 +324,20 @@ Data* data_decompress_limited(Data* compressed_data, size_t maximum_size) {
       uncompressed_data->data = new_data;
       output.dst = new_data;
       output.size = buf_size;
+      /* Re-attempt with the larger output buffer; the truncated-frame check
+       * below must not reject a complete frame that merely filled the previous
+       * buffer exactly. */
+      continue;
+    }
+    /* A positive hint with all input consumed means the frame is incomplete: a
+     * truncated stream would otherwise spin here forever (ZSTD_decompressStream
+     * keeps returning the same hint).  Fail instead of burning CPU. */
+    if (ret != 0 && input.pos == input.size) {
+      log_message(LOG_LEVEL_ERROR,
+                  "Truncated zstd frame: input exhausted with %zu bytes still expected", ret);
+      data_destroy(uncompressed_data);
+      uncompressed_data = NULL;
+      goto cleanup;
     }
   } while (ret > 0);