Make flattened synthetic source concatenate object keys on scalar/object mismatch (elastic#129600)

There is an issue where for Flattened fields with synthetic source, if there is a key with a scalar value, and a duplicate key with an object value, one of the values will be left out of the produced synthetic source. This fixes the issue by replacing the object with paths to each of its keys. These paths consist of the concatenation of all keys going down to a given scalar, joined by a period. For example, they are of the form foo.bar.baz. This applies recursively, so that every value within the object, no matter how nested, will be accessible through a full specified path.

Author: parkertimmins

docs/changelog/129600.yaml

@@ -0,0 +1,7 @@
+pr: 129600
+summary: Make flattened synthetic source concatenate object keys on scalar/object
+  mismatch
+area: Mapping
+type: bug
+issues:
+ - 122936

docs/reference/elasticsearch/mapping-reference/flattened.md

@@ -364,3 +364,36 @@ Will become (note the nested objects instead of the "flattened" array):
   }
 }
 ```
+
+Flattened fields allow for a key to contain both an object and a scalar value.
+For example, consider the following flattened field `flattened`:
+
+```console-result
+{
+  "flattened": {
+    "foo.bar": "10",
+    "foo": {
+      "bar": {
+        "baz": "20"
+      }
+    }
+  }
+}
+```
+
+Because `"foo.bar": "10"` is implicitly equivalent to `"foo": { "bar": "10" }`,
+`"bar"` has both a scalar value `"10"`, and an object value of `{ "baz": "20" }`.
+
+With synthetic source, to produce a valid JSON output, objects with such fields will appear differently in `_source`.
+For example, if the field is defined in an index configured with synthetic source, the value of `_source` would be:
+
+```console-result
+{
+  "flattened": {
+    "foo": {
+      "bar": "10",
+      "bar.baz": "20"
+    }
+  }
+}
+```