Remove proptest dev-dependency in favor of manual randomization

[tnull]

A sub-dependency of proptest started depending on getrandom v0.4.1 which uses edition = 2024, which isn't supported in our MSRV of 1.75.0.

Here we replace the proptest-based round-trip test with a simple loop using rand (already a regular dependency) to generate random inputs. This removes a dev-dependency and its associated MSRV pin in CI.

Co-Authored-By: HAL 9000

[ldk-reviews-bot]

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[tankyleo]

nit: if we have \t, let's also add \n and \r ? HAL tells me these would be matched by the earlier regex given \s

[tnull]

Alright, now amended to include (though I doubt it makes any difference, in practice our charset should be much more restrictive anyways):

diff --git a/src/util/key_obfuscator.rs b/src/util/key_obfuscator.rs
index 4e7ade2..c096e3d 100644
--- a/src/util/key_obfuscator.rs
+++ b/src/util/key_obfuscator.rs
@@ -200,5 +200,5 @@ mod tests {

                let charset =
-                       b"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_!@#,;:% *$^&()[]{}.\t";
+                       b"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_!@#,;:% *$^&()[]{}.\t\n\r";
                let mut rng = rand::thread_rng();

[tnull]

Want to note that it would be nice to drop this pin at some point, now that we don't depend on url anymore. However, we do still depend on it through mockito. Given we previously wondered how much benefit the mock testing gives us to begin with, we might at some point consider to drop that dev-dependency, too.