fix: derive a valid package for shallow export model URIs in ExportGeneratorX

[joaodinissf]

Summary

ExportGeneratorX.getPrefix derives the target package from raw URI segment arithmetic and misbehaves on every short URI shape:

• 1 segment (e.g. ParseHelper's __synthetic0.export): subList(-1, 0) → IndexOutOfBoundsException.
• 2 segments: emits the literal segment resource as the package.
• project root (platform:/resource/<project>/<file>.export): emits the raw project name — uncompilable generated code when the project is not named like a Java package (e.g. My-Project).
• source-folder root (<project>/src/<file>.export): emits an empty package, producing leading-dot qualified names (.naming.…).

The derived value feeds both the generated package declaration and the output file path via the six accessor methods (ExportedNamesProvider, ResourceDescriptionManager/Strategy/Constants, FingerprintComputer, FragmentProvider).

The fix is confined to that one method:

• URIs with five or more segments keep the previous derivation byte for byte (segments between position 3 and the file, regardless of the source folder's name) — no behavior change for any layout that worked before, including src-gen nesting and src/main/java.
• Shorter shapes fall back to the project name when it already is a valid Java package name (SourceVersion.isName, byte-identical to previous behavior for the project-root shape), or generated otherwise.

No existing test ever failed on this: the builder participant catches the exception per resource and logs it — the long-standing ERROR … Error during compilation of '….export' noise in every master test run — and the export tests assert validation/scoping, never generated output (that structural gap is addressed separately by #1459). The new ExportGeneratorXTest pins the shape table with synthetic resource URIs; no fixture files needed.

Verification

• Full local gate on this head: mvn -T 3C clean verify checkstyle:check pmd:check pmd:cpd-check spotbugs:check → BUILD SUCCESS, 0 test failures, including the new ExportGeneratorXTest (runs in CI via ExportTestSuite → AllTests).
• A/B: the new tests grafted onto plain master (production code unchanged) fail on exactly the broken shapes — IndexOutOfBoundsException: fromIndex = -1 for the single-segment URI and wrong/leading-dot packages for the short shapes — while the behavior-preservation tests (nested derivations, valid project-name fallback) pass on both master and this branch.

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