WW-5630 Streamline ConfigParseUtilTest and convert to JUnit 4 (6.x)

[lukaszlenart]

Port of #1740 to support/struts-6-x-x.

Test-only cleanup of ConfigParseUtilTest (added on 6.x via #1736, cherry-pick of #1721):

• Collapse redundancy: 12 overlapping cache tests → 5 focused ones (single-loader caching, per-loader isolation incl. the same-toString() case, outer-size bound, inner-size bound, and the negative/not-cached case).
• Drop the ~80-entry literal: the inner-cache size-bound test now generates synthetic class names in a loop bounded by the per-loader limit (read via the existing helper) instead of hardcoding a giant JDK class-name array.
• Remove unnecessary reflection: the behavioral tests assert via load counts only; reflection is retained solely in the two size-bound tests, where Caffeine exposes no public seam for estimatedSize().
• JUnit 3 → JUnit 4: @Test/@Before/@After with static org.junit.Assert.*, matching the convention of newer core unit tests (e.g. NamedVariablePatternMatcherTest).
• Also reorders the caffeine imports in ConfigParseUtil.java to alphabetical (matching WW-5630 - Performance Issue SecurityMemberAccess #1721 on main).

Production caching logic is unchanged.

Test Plan

• ./mvnw -pl core clean test -Dtest=ConfigParseUtilTest → Tests run: 5, Failures: 0, Errors: 0, Skipped: 0
• ./mvnw -pl core apache-rat:check → BUILD SUCCESS

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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