pythongh-150583: Fix zstd compression level with digested ZstdDict (python#150586)

Author: Rogdham

Misc/NEWS.d/next/Library/2026-06-03-19-35-32.gh-issue-150583.dedI24.rst

@@ -0,0 +1,2 @@
+Correctly set the default compression level in :mod:`compression.zstd` when passing a digested
+dictionary during compression.

Modules/_zstd/compressor.c

@@ -347,6 +347,7 @@ _zstd_ZstdCompressor_new_impl(PyTypeObject *type, PyObject *level,
     }
 
     self->use_multithread = 0;
+    self->compression_level = ZSTD_CLEVEL_DEFAULT;
     self->dict = NULL;
     self->lock = (PyMutex){0};