Fix/doi access free pr

[redalert2fan]

Summary

• Avoid adding duplicate doi-access=free parameters.
• Upgrade existing non-free DOI access values when the DOI is known to be free.
• Add regression tests for both cases.
• Use mb_substr_count() in tests to satisfy PHPCS.

Test Plan

• Focused DOI access tests pass: 5 tests, 9 assertions.
• Changed files are limited to Template.php and templatePart4Test.php.

[redalert2fan]

test fail unrelated to this patch, should be fixed by #5716