fix: replace rusb GlobalContext with explicit Context to prevent panics

[zbowling]

Summary

• Replace all uses of rusb::devices() (which internally uses GlobalContext and panics via Once::call_once if libusb_init fails) with explicit rusb::Context::new() + context.devices() which returns a Result instead
• Replace all GlobalContext type references with Context in USBTransport struct and method signatures
• This makes the crate usable in environments without USB support (CI servers, containers, etc.) by returning errors instead of panicking

Test plan

• cargo clippy passes with no warnings
• cargo test passes (5 unit tests + 4 doc-tests)
• Manual testing on a system without libusb installed to confirm error is returned instead of panic

[zbowling]

This is fixing an issue with some of my tests in CI where libusb doesn't exist.

[cocool97]

Thanks for your PR ! :)