MATH-1688: Reject malformed grouping separators in ComplexFormat

[efegokdemir]

Summary

Fixes MATH-1688 by preventing the default ComplexFormat parser from silently accepting malformed grouping separators as part of numeric values.

Changes

• Disable grouping separators in the default number formats created by ComplexFormat.
• Add regression coverage for malformed comma placement reported in MATH-1688.
• Custom NumberFormat instances remain unchanged.

Testing

• mvn -pl commons-math-legacy -am -Dtest=ComplexFormatTest -Dsurefire.failIfNoSpecifiedTests=false -Djacoco.skip=true test — passed (44 tests).
• git diff --check — passed.
• mvn -pl commons-math-legacy -am -DskipTests -Djacoco.skip=true verify — blocked by the repository's SpotBugs plugin, which does not support the JDK 27 class-file version (Unsupported class file major version 71).

Notes

The issue was reproduced on the current upstream branch before the change: malformed inputs such as ,,7+,,,2i and 7,,8 were accepted. The fix is limited to ComplexFormat defaults and does not alter caller-supplied number formats.

AI assistance was used during investigation and implementation; the change was reviewed and validated by the submitter.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.17%. Comparing base (f554608) to head (4d664da).
⚠️ Report is 350 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #330      +/-   ##
============================================
+ Coverage     86.54%   87.17%   +0.63%     
+ Complexity     9787       89    -9698     
============================================
  Files           532      499      -33     
  Lines         35516    33469    -2047     
  Branches       6194     5834     -360     
============================================
- Hits          30738    29178    -1560     
+ Misses         3518     3165     -353     
+ Partials       1260     1126     -134     

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[aherbert]

This change does not pass the code style checks.

Please run this using a JDK lower than 27 so you can use the default maven goal and pass all the checks. It should pass with:

mvn -pl commons-math-legacy
# if the build is flaky you can increase the test count from the default of 5, e.g
mvn -pl commons-math-legacy -Dsurefire.rerunFailingTestsCount=10

[efegokdemir]

Applied the requested style correction in commit 3b39724: ComplexFormat declarations now follow the repository Checkstyle declaration order (static field, instance fields, constructors, then helper methods).

Validation with JDK 21.0.12.1:

• mvn -pl commons-math-legacy: Checkstyle passes; the build remains red only because the existing legacy suite has intermittent numerical/optimization failures.
• mvn -pl commons-math-legacy -Dsurefire.rerunFailingTestsCount=10: same unrelated suite failure remained after reruns.
• Focused ComplexFormatTest,ComplexFormatAbstractTest: 44 tests passed.
• git diff --check: passed.

The correction commit is SSH-signed and DCO-signed. Please re-check the PR when convenient.