🧪 Add missing test for std.pdf module registration

[Tcode-Motion]

🎯 What: The stdlib/src/pdf.rs lacked tests verifying its module registration functionality.
📊 Coverage: A new unit test test_register_pdf is added that verifies if std.pdf is correctly registered with its intended properties, ensuring future modifications don't break module initializations.
✨ Result: Increased testing coverage on standard library module setup routines, bringing it to parity with other standard modules like json and compress.

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PR created automatically by Jules for task 12482997513856093243 started by @Tcode-Motion

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