Add csv test for LU JOIN with union types

Author: alex-spies

x-pack/plugin/esql/qa/testFixtures/src/main/resources/lookup-join.csv-spec

@@ -732,6 +732,28 @@ language.id:integer | language.name:text | language.name.keyword:keyword | langu
 2                   | French             | French                        | FR
 ;
 
+###############################################
+# union type behavior
+###############################################
+
+joinOnMultiTypedMatchFieldCastToInteger
+required_capability: join_lookup_v12
+
+FROM apps, apps_short METADATA _index
+| EVAL language_code = id::integer
+| KEEP _index, language_code
+| WHERE language_code < 3
+| LOOKUP JOIN languages_lookup ON language_code
+| SORT _index ASC, language_code ASC
+;
+
+_index:keyword | language_code:integer | language_name:keyword
+apps           | 1                     | English
+apps           | 2                     | French
+apps_short     | 1                     | English
+apps_short     | 2                     | French
+;
+
 ###############################################
 # Tests with clientips_lookup index
 ###############################################