GH-4300: Compare SPARQL strings by Unicode code point - #4314

GitHub issue resolved #4300

Pull request Description:

SPARQL uses the Unicode code point collation for string comparisons. XSDFuncOp.compareString used String.compareTo, which compares UTF-16 code units. For example, U+FFFD should sort before U+1D11E, but their UTF-16 order is reversed. This affects string </> comparisons and the value-comparison path used for sorting.

Compare one code point at a time, while retaining the existing behavior for equal strings and prefixes. Add regressions for plain and xsd:string expression comparisons and NodeValueSortKey ordering. The change is confined to three files and adds no dependency or public API.

This follows the SPARQL 1.1 string collation and the Unicode Codepoint Collation.

Validation:

• On main at d77438478938cc651760f4a61e5d9ee7acff072b, both new regression tests failed. A separate actual-SPARQL probe using U+FFFD, U+FFFE and U+1D11E failed all six checks for <, >, ORDER BY, MIN() and MAX(); the old order was U+1D11E, U+FFFD, U+FFFE.
• With this change, the two new regression tests and all six actual-SPARQL checks passed. The SPARQL probe is a pre-submit validation artifact outside the three-file PR diff.
• mvn -B -q -pl jena-arq -am -Dtest=TestGH4300RealSPARQL,TestExpressions2,TestNodeValueSortKey,TestNodeValueOps,TestOrdering,TestSortOrdering,TestComparison,TestStatisticsAggregates,TestNodeValue,TestCustomAggregates -Dsurefire.failIfNoSpecifiedTests=false test passed: 401 tests including the temporary probe, 0 failures/errors/skips, using Java 21 in an isolated Maven container. The prior 198-test subset remains green.
• git diff --check passed.

The full Jena test suite was not run.

• Tests are included.
• Documentation change and updates are provided for the Apache Jena website (not needed for this bug fix).
• Commits have been squashed to remove intermediate development commit messages (local changes are not committed yet).
• Key commit messages start with the issue number (to complete before submission).

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See the Apache Jena "Contributing" guide.