🧪 Add unit test for PDF module registration

[Tcode-Motion]

🎯 What: The testing gap addressed
Added a missing unit test for the register_pdf standard library module registration function within stdlib/src/pdf.rs.

📊 Coverage: What scenarios are now tested
The unit test validates the happy path of module registration. It verifies that invoking register_pdf on a new StdlibRegistry correctly provisions the module with the expected name ("std.pdf"), version ("1.0.0"), and empty sets for both exports and required capabilities.

✨ Result: The improvement in test coverage
Ensures that future modifications to the PDF module registration or underlying registry mechanism will not silently drop or misconfigure the std.pdf module.

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PR created automatically by Jules for task 15255348215550307609 started by @Tcode-Motion

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