…c#89389)

When parsing mappings, we may find a field with same name specified twice, either
because JSON duplicate keys are allowed, or because a mix of object notation and dot
notation is used when providing mappings. The same can happen when applying dynamic
mappings as part of parsing an incoming document, as well as when merging separate
index templates that may contain the definition for the same field using a
mix of object notation and dot notation.

While we propagate the MapperBuilderContext across merge calls thanks to elastic#86946, we do not
propagate the right context when we call merge on objects as part of parsing/building
mappings. This causes a situation in which the leaf fields that result from the merge
have the wrong path, which misses the first portion e.g. sub.field instead of obj.sub.field.

This commit applies the correct mapper builder context when building the object mapper builder
and two objects with same name are found.

Relates to elastic#86946

Closes elastic#88573