Check str constraints on dict keys in msgpack

Previously the msgpack decoder would miss checking constraints on dict
keys if those keys were short ascii strings. This fixes that and adds a
test.

msgspec/_core.c

@@ -14691,11 +14691,9 @@ mpack_decode_key(DecoderState *self, TypeNode *type, PathNode *path) {
     /* Peek at the next op */
     op = *self->input_pos;
 
-    if (MS_LIKELY(
-            '\xa0' <= op && op <= '\xbf' &&
-            type->types & (MS_TYPE_STR | MS_TYPE_ANY)
-        )
-    ) {
+    bool is_str = type->types == MS_TYPE_ANY || type->types == MS_TYPE_STR;
+
+    if (MS_LIKELY(is_str && '\xa0' <= op && op <= '\xbf')) {
         /* A short (<= 31 byte) unicode str */
         self->input_pos++; /* consume op */
         Py_ssize_t size = op & 0x1f;

tests/test_constraints.py

@@ -549,6 +549,23 @@ class Ex(msgspec.Struct):
             with pytest.raises(msgspec.ValidationError):
                 dec.decode(proto.encode(Ex(x)))
 
+    @pytest.mark.parametrize(
+        "meta, good, bad",
+        [
+            (Meta(min_length=2), ["xy", "𝄞xy"], ["", "𝄞"]),
+            (Meta(pattern="as"), ["as", "pass", "𝄞as"], ["", "nope", "𝄞"]),
+        ],
+    )
+    def test_str_constraints_on_dict_keys(self, proto, meta, good, bad):
+        dec = proto.Decoder(Dict[Annotated[str, meta], int])
+
+        for x in good:
+            assert dec.decode(proto.encode({x: 1})) == {x: 1}
+
+        for x in bad:
+            with pytest.raises(msgspec.ValidationError):
+                dec.decode(proto.encode({x: 1}))
+
 
 class TestDateTimeConstraints:
     @staticmethod