fix(mongo): replace full-document Save instead of $set merge

[ZhuchkaTriplesix]

Summary

• Full-document JSON Save uses replaceOne via MongoService.replaceDocument, so keys deleted in the editor (including nested maps) are removed on the server.
• _id stays the original BSON value even if the JSON changed it. Inspector single-field $set is unchanged.

Closes #778

Test plan

• Open a document {a:1,b:2}, delete b in JSON, Save → reload shows {a:1} only
• Nested: {nested:{x:1,y:2}} → save {nested:{x:1}} → y is gone
• Changing _id in JSON does not rewrite the document id
• Inspector Save to DB still $sets one field