Don't ignore additional entries in UntypedReflectDeserializerVisitor (#7112)

# Objective

Currently when `UntypedReflectDeserializerVisitor` deserializes a
`Box<dyn Reflect>` it only considers the first entry of the map,
silently ignoring any additional entries. For example the following RON
data:

```json
{
    "f32": 1.23,
    "u32": 1,
}
```

is successfully deserialized as a `f32`, completly ignoring the `"u32":
1` part.

## Solution

`UntypedReflectDeserializerVisitor` was changed to check if any other
key could be deserialized, and in that case returns an error.

---

## Changelog

`UntypedReflectDeserializer` now errors on malformed inputs instead of
silently disgarding additional data.

## Migration Guide

If you were deserializing `Box<dyn Reflect>` values with multiple
entries (i.e. entries other than `"type": { /* fields */ }`) you should
remove them or deserialization will fail.

crates/bevy_reflect/src/serde/de.rs

@@ -8,7 +8,8 @@ use crate::{
 };
 use erased_serde::Deserializer;
 use serde::de::{
-    self, DeserializeSeed, EnumAccess, Error, MapAccess, SeqAccess, VariantAccess, Visitor,
+    self, DeserializeSeed, EnumAccess, Error, IgnoredAny, MapAccess, SeqAccess, VariantAccess,
+    Visitor,
 };
 use serde::Deserialize;
 use std::any::TypeId;
@@ -322,11 +323,17 @@ impl<'a, 'de> Visitor<'de> for UntypedReflectDeserializerVisitor<'a> {
     {
         let registration = map
             .next_key_seed(TypeRegistrationDeserializer::new(self.registry))?
-            .ok_or_else(|| Error::invalid_length(0, &"at least one entry"))?;
+            .ok_or_else(|| Error::invalid_length(0, &"a single entry"))?;
+
         let value = map.next_value_seed(TypedReflectDeserializer {
             registration,
             registry: self.registry,
         })?;
+
+        if map.next_key::<IgnoredAny>()?.is_some() {
+            return Err(Error::invalid_length(2, &"a single entry"));
+        }
+
         Ok(value)
     }
 }