NIFI-15649 - Add Expression Language functions compactDelimitedList() and trimDelimitedList() for delimited string cleanup

• compactDelimitedList removes all empty tokens from delimited strings
• trimDelimitedList removes only leading/trailing empty tokens, preserving interior empties
• Handles multi-character delimiters
• Added unit tests in TestQuery
• Updated ANTLR grammar (lexer and parser)
• Integrated into ExpressionCompiler
• Added documentation to expression language guide

Summary

NIFI-15649

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Build

• Build completed using ./mvnw clean install -P contrib-check
  • JDK 21
  • [] JDK 25

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