`FilterExpressionTextParser` fails to parse expressions whose first key starts with the letters `where`

Bug description

FilterExpressionTextParser prepends the compulsory WHERE keyword only when the expression does not already start with it, but the check is a plain string prefix test:

if (!textFilterExpression.toUpperCase(Locale.ROOT).startsWith(WHERE_PREFIX)) {

A metadata key that merely starts with the letters where — such as whereabouts or where_clause — satisfies the prefix check, so WHERE is never prepended. The lexer then tokenizes the identifier as IDENTIFIER rather than the WHERE token required by the grammar rule where : WHERE booleanExpression EOF, and parsing fails.

Environment

• Spring AI 2.0.2-SNAPSHOT, main at 0933bd0
• Module: spring-ai-vector-store
• Class: org.springframework.ai.vectorstore.filter.FilterExpressionTextParser

Steps to reproduce

new FilterExpressionTextParser().parse("whereabouts == 'BG'");

fails with:

FilterExpressionTextParser$FilterExpressionParseException
Caused by: org.antlr.v4.runtime.InputMismatchException
	at org.springframework.ai.vectorstore.filter.antlr4.FiltersParser.where(FiltersParser.java:186)

while e.g. parse("country == 'BG'") works.

Expected behavior

whereabouts == 'BG' parses to EQ(Key("whereabouts"), Value("BG")) like any other key. The prefix check should only match the standalone WHERE keyword (e.g. ^\s*where\b, case-insensitive), not any identifier that happens to start with those letters.

Start in the spring-ai-vector-store module at org.springframework.ai.vectorstore.filter.FilterExpressionTextParser and reproduce the failure with whereabouts == 'BG'. Check the existing WHERE-prefix handling and its interaction with the FiltersParser grammar. Done means keys beginning with where parse successfully while the standalone WHERE keyword continues to work.