fix: prevent dictionary mutation during iteration in preprocess_schema

When processing schema definitions with block_type_slug where the
definition's title differs from the dict key, the code would add a new
key to the definitions dict while iterating over it, causing:
    RuntimeError: dictionary changed size during iteration

This fix iterates over a snapshot of the dict values using list() to
allow safe modification during iteration.

Fixes #19935

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: zzstoatzz

src/prefect/utilities/schema_tools/validation.py

@@ -257,7 +257,7 @@ def preprocess_schema(
 
     if "definitions" in schema:  # Also process definitions for reused models
         definitions = cast(dict[str, Any], schema["definitions"])
-        for definition in definitions.values():
+        for definition in list(definitions.values()):
             if "properties" in definition:
                 required_fields = definition.get("required", [])
                 process_properties(

tests/utilities/schema_tools/test_validation.py

@@ -1977,6 +1977,42 @@ class MyModel(BaseModel):
         }
 
 
+class TestPreprocessSchemaBlockTypes:
+    """Regression tests for block type handling in preprocess_schema.
+
+    See: https://github.com/PrefectHQ/prefect/issues/19935
+    """
+
+    def test_block_type_slug_with_mismatched_title_does_not_raise(self):
+        """When a definition has block_type_slug and the dict key differs from
+        the title, preprocess_schema should not raise RuntimeError.
+
+        This bug occurred because the code modified schema["definitions"] while
+        iterating over it - adding a new key when title != dict key.
+        """
+        schema = {
+            "properties": {
+                "my_block": {"$ref": "#/definitions/prefect__blocks__system__Secret"}
+            },
+            "definitions": {
+                # Key is the full module path, but title is just "Secret"
+                "prefect__blocks__system__Secret": {
+                    "title": "Secret",  # Different from the dict key!
+                    "block_type_slug": "secret",
+                    "type": "object",
+                    "properties": {"value": {"type": "string"}},
+                }
+            },
+        }
+
+        # This should not raise RuntimeError: dictionary changed size during iteration
+        result = preprocess_schema(schema)
+
+        # The schema should be transformed to include oneOf for block reference
+        assert "Secret" in result["definitions"]
+        assert "oneOf" in result["definitions"]["Secret"]
+
+
 class TestPreprocessSchemaPydanticV2Tuples:
     # We should always be conforming to PydanticV2 and there should
     # be no unintended changes here.