fix: Bug: intermittent empty highlights with allow_empty=true on distributed ...

[Roshan931]

Fixes #4769

Root cause

Merged distributed result set evaluates HIGHLIGHT/SNIPPET with one table's expression

Changes

• In minimize_aggr_result.cpp the post-limit expressions of every local result set are now collected per tag right after tags are compacted (while the per-table schemas are still in place) and ProcessMultiPostlimit evaluates each merged match with the expression belonging to the table the match came from, falling back to the merged-schema expression when no per-tag one exists. Added a CLT regression test with a distributed table over two locals with different morphology.

[klirichek]

in description of the PR you've promised a test.
In that very case even ubertest is perfect solution, since the bug should be easy reproduced on a few modeling data (i.e. no need to setup/construct MCL test with startup logic, non-trivial console tricks, etc).

Actually only one file changed; no test provided.
The change is not trivial typo correction somewhere in the comments, so, it NEED to be tested.