fix: Don't assume homogeneous data in meta tables

Steps to reproduce:
- enable developer mode (doesn't happen in prod)
- Save a document with set only once fields
- Reload the page (requests meta again which is now polluted)

This is new category of bug surfaced because meta objects now live
longer than request and all kinds of weird `self._cached_property`
starts getting serialized.

Co-Authored-By: ruthra kumar <ruthra@erpnext.com>

frappe/core/doctype/doctype/test_doctype.py

@@ -796,6 +796,14 @@ class TestDocType(IntegrationTestCase):
 		)
 		self.assertRaises(frappe.ValidationError, recursive_dt.insert)
 
+	def test_meta_serialization(self):
+		doctype = new_doctype(
+			fields=[{"fieldname": "some_fieldname", "fieldtype": "Data", "set_only_once": 1}]
+		).insert()
+		doc = frappe.new_doc(doctype.name, some_fieldname="something").insert()
+		doc.save()
+		frappe.get_meta(doctype.name).as_dict()
+
 
 def new_doctype(
 	name: str | None = None,

frappe/model/meta.py

@@ -188,6 +188,8 @@ class Meta(Document):
 
 	def as_dict(self, no_nulls=False):
 		def serialize(doc):
+			if isinstance(doc, dict):
+				return doc.copy()
 			out = {}
 			for key, value in doc.__dict__.items():
 				if isinstance(value, list | tuple):