feat: support zonemap indexes in ALTER TABLE CREATE INDEX

[beinan]

Summary

• add single-column zonemap support for ALTER TABLE ... CREATE INDEX via Lance direct createIndex instead of fragment training
• surface zonemap indexes in SHOW INDEXES and scan planning, and fix numeric zonemap pruning across mixed numeric types
• clarify and test that multi-column zonemap is not yet supported, with a clearer Spark-side error and updated docs

Testing

• ./mvnw -pl lance-spark-4.0_2.13,lance-spark-4.1_2.13 -Dtest=AddIndexTest,ShowIndexesTest,ZonemapFragmentPrunerTest,CreateIndexStandardSyntaxTest -Dsurefire.failIfNoSpecifiedTests=false test
• ./mvnw -pl lance-spark-4.0_2.13,lance-spark-4.1_2.13 -Dtest=AddIndexTest,ShowIndexesTest -Dsurefire.failIfNoSpecifiedTests=false test

Notes

• generic Spark CREATE INDEX syntax accepts a column list, but current Lance core rejects multi-column zonemap creation with LanceError(Index): Only support building index on 1 column at the moment
• Spark now fails earlier with Zonemap index currently supports a single column only
• Python integration tests were updated but not run locally because this environment does not have pytest or the expected /home/lance/data fixture path