Fix function which generates test field names

[seanzatzdev]

One of the methods used for generating field paths in tests has a bug that can occur which will generate empty path segments due to randomly generated strings that include . values.

e.g.

1> [2025-10-17T07:05:23,431][INFO ][o.e.i.c.AppendProcessorTests][testAppendValuesToExistingList] [seed=[622839918FFAD261:E8D31C04FD5E16CA]] after test
  2> REPRODUCE WITH: ./gradlew ":modules:ingest-common:test" --tests "org.elasticsearch.ingest.common.AppendProcessorTests.testAppendValuesToExistingList {seed=[622839918FFAD261:E8D31C04FD5E16CA]}" -Dtests.seed=622839918FFAD261 -Dtests.locale=sa -Dtests.timezone=Pacific/Kanton -Druntime.java=25
  2> java.lang.IllegalArgumentException: path [𹡹.MC..񗃹𝮠𚰉񈌇򂦕򆿰.7VQ] is not valid
        at __randomizedtesting.SeedInfo.seed([622839918FFAD261:E8D31C04FD5E16CA]:0)
        at org.elasticsearch.ingest.IngestDocument$FieldPath.validateClassicFields(IngestDocument.java:1412)
        at org.elasticsearch.ingest.IngestDocument$FieldPath.processPathParts(IngestDocument.java:1397)
        at org.elasticsearch.ingest.IngestDocument$FieldPath.<init>(IngestDocument.java:1392)
        at org.elasticsearch.ingest.IngestDocument$FieldPath.of(IngestDocument.java:1366)
        at org.elasticsearch.ingest.IngestDocument.setFieldValue(IngestDocument.java:754)
        at org.elasticsearch.ingest.IngestDocument.setFieldValue(IngestDocument.java:694)
        at org.elasticsearch.ingest.RandomDocumentPicks.addRandomField(RandomDocumentPicks.java:108)
        at org.elasticsearch.ingest.common.AppendProcessorTests.testAppendValuesToExistingList(AppendProcessorTests.java:50)
  2> NOTE: leaving temporary files on disk at: /Users/seanzatz/workspace/elasticsearch/modules/ingest-common/build/testrun/test/temp/org.elasticsearch.ingest.common.AppendProcessorTests_622839918FFAD261-001
  2> NOTE: test params are: codec=Asserting(Lucene103): {}, docValues:{}, maxPointsInLeafNode=774, maxMBSortInHeap=6.044067774802311, sim=Asserting(RandomSimilarity(queryNorm=true): {}), locale=sa, timezone=Pacific/Kanton
  2> NOTE: Mac OS X 15.7.1 aarch64/Oracle Corporation 25 (64-bit)/cpus=12,threads=1,free=425678512,total=536870912
  2> NOTE: All tests run in this JVM: [AppendProcessorTests]

This PR updates the method logic to only generate valid paths via using a method which excludes . values

Closes #136701

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[szybia]

nice!

[joegallo]

Thanks for iterating with us. Ship it!