A bug for generating testnet descriptor?

[eauxxs]

Describe the bug
I want a testnet descriptor key, but get a mainnet descriptor key.

To Reproduce

#[test]
fn test_override() {
    let xpriv: GeneratedKey<Xpriv, miniscript::Segwitv0> = Xpriv::generate(()).unwrap();
    let a =  xpriv.into_descriptor_key(None, "m/84h/1h/0h".parse().unwrap()).unwrap();
    let a = a.override_valid_networks(test_networks());
    let secp = SecpCtx::new();
    println!("{}", a.extract(&secp).unwrap().0);
    // [be3917e7/84'/1'/0']xpub6BxR3GaY9SrwFUWTRnmu3qyit4GECypBqNJPycFbFNGLkkh4Bjszb2LGqUbuF1ogEfCTvLo8V3DwPtuqcG4An2h34LQywRmUusRFErevX4L/*
}

Expected behavior
[fingerprint/derivepath]tpubxxxxx/*

Build environment

• BDK tag/commit:
• OS+version:
• Rust/Cargo version:
• Rust/Cargo target:

Additional context
https://github.com/bitcoindevkit/bdk/blob/9af0fd4d1d039b94caeecf290897006868dbe7f6/crates/wallet/src/keys/mod.rs#L716-L718

I don't know if this flow is correct, but reading it literally it should produce a testnet descriptor. So I'm not sure if this is a bug or not.

[ValuedMammal]

To generate a tpub look at the tutorials in Book of BDK, check the example code snippet mnemonic_to_descriptors.rs, or try using bdk-cli.

override_valid_networks is really for special cases where the network is not already encoded by the secret key.

However it's not intuitive to me why in the above example bdk says we support any network.