Nested path info shouldn't be added during copy_to

[romseygeek]

When executing the copy_to mappings on a nested doc, if the location was
underneath another nested mapper then the document parser could end up
adding a new nested path metadata field for the source to the destination
document. This was mostly ignored, but could make calculation of a
NestedIdentity incorrect, leading to exceptions when loading the source of
a nested document during the fetch phase.

This commit moves all of the nested path handling directly into
DocumentParserContext.createNestedContext(), which already has some
logic to detect if we're in a copy_to context.

Fixes #93117

[elasticsearchmachine]

Pinging @elastic/es-search (Team:Search)

[elasticsearchmachine]

Hi @romseygeek, I've created a changelog YAML for you.

[romseygeek]

@elasticmachine update branch

[javanna]

LGTM, can you double check that the additional tests are enough? Do they cover the scenario that the user reported? it was not obvious to me.

[romseygeek]

I did have another test that used the input from the original bug, but after I'd narrowed everything down sufficiently to reproduce with a simple test it turned out that a couple of simple checks on the existing CopyToMapperTests did the same thing. The bug would manifest itself as a nested doc containing multiple _nested_path fields, so checking that each nested field just contains a single instance of this field is sufficient to catch a regression.

[javanna]

Thanks for clarifying @romseygeek