Recalculate delimiter reader mark length - #386

Summary

Fixes #354 by recalculating the reader mark length whenever delimiter specifications are replaced.

Changes

• Reuse calculateMarkLength() after clearing and parsing the current delimiter set.
• Add a regression test that calls setDelimiterSpecs() repeatedly and verifies the mark length does not accumulate.

Testing

• JAVA_HOME=/opt/homebrew/opt/openjdk@21 mvn test — PASS (76 tests).
• JAVA_HOME=/opt/homebrew/opt/openjdk@21 mvn verify — PASS (76 tests; Checkstyle, Spotless and RAT passed).
• JAVA_HOME=/opt/homebrew/opt/openjdk@21 mvn -Prun-its verify — PASS, with Maven reporting that the repository does not define a run-its profile.
• git diff --check — PASS.

Notes

• Final duplicate check found no open PR for setDelimiterSpecs() accumulates markLength instead of recalculating #354 or the mark-length delimiter change.
• The commit is signed and includes the required DCO sign-off.
• AI assistance was used to prepare this change; the submitter reviewed the implementation and test results.