🧪 [testing improvement] Add missing test for module registration in jwt.rs

[Tcode-Motion]

🎯 What: The testing gap addressed was a missing unit test for the module registration logic inside stdlib/src/jwt.rs.
📊 Coverage: The scenarios now tested involve the proper setup and registration of the std.jwt module by the StdlibRegistry. The new test specifically verifies that the module is correctly attached, its exported functions (encode and decode) have the correct arity, correct names, and behave properly when called with arguments (returning dummy/placeholder data).
✨ Result: Test coverage for the standard library's JWT module setup is now verified and safe from regression.

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PR created automatically by Jules for task 13250582641171105321 started by @Tcode-Motion

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