Rust panic when passed endpoint with a space in it

Just FYI I managed to cause a rust panic just now by accidentally passing a http endpoint string with a space at the end of it. Seems like something that could be caught more gracefully.

No source file or test is named. Reproduce the panic with an HTTP endpoint ending in a space, trace the Rust endpoint-validation path, and identify the expected graceful error behavior; done means the input no longer panics and a regression test covers it.

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