PDFBOX-6286: Scope page-tree recursion state to each lookup

[AryanRG]

Summary

• Fix false recursion on a valid PDPageTree.get() lookup after an out-of-range lookup on the same tree by scoping the visited set to each traversal.
• Add an in-memory, one-page regression test for the failing sequence.

Jira: https://issues.apache.org/jira/browse/PDFBOX-6286

Validation

• TestPDPageTree: 8 tests passed using focused Maven resource, compile, and test goals.
• The standard Maven test lifecycle did not finish while fetching external fixtures. The full suite and stable branches were not tested.

Prepared with AI assistance.