`KeychainTxOutIndex::insert_descriptor` panics on unsupported derivation paths

**Describe the bug**

Syntactically valid descriptors can parse successfully and then panic inside `KeychainTxOutIndex::insert_descriptor`, before it returns its `Result`:

- `wpkh(/0/*')` and `wpkh(/0'/1)`: miniscript panics with `HardenedStep`.
- `wpkh(/<0;1>/*)`: `descriptor_id()` unwraps `Err(Multipath)`.

`insert_descriptor` computes `descriptor_id()` before validating these derivation forms. Callers cannot handle the failed insertion through the returned `Result`.

This issue was found by AI.

**To Reproduce**

Add this test to `crates/chain/tests/test_keychain_txout_index.rs` and run `cargo test -p bdk_chain --test test_keychain_txout_index insert_descriptor_does_not_panic_on_hardened_wildcard`. Reproduced on `acc06e53220960caa89efd5984d7b43914640dd4` with miniscript 13.1.0.

```rust
#[test]
fn insert_descriptor_does_not_panic_on_hardened_wildcard() {
use bdk_chain::{
keychain_txout::KeychainTxOutIndex,
miniscript::{Descriptor, DescriptorPublicKey},
};

const XPUB: &str = "tpubD6NzVbkrYhZ4XgiXtGrdW5XDAPFCL9h7we1vwNCpn8tGbBcgfVYjXyhWo4E1xkh56hjod1RhGjxbaTLV3X4FyWuejifB9jusQ46QzG87VKp";
let descriptor: Descriptor =
format!("wpkh({XPUB}/0/*')").parse().unwrap();
let mut index = KeychainTxOutIndex::::new(1, true);
let _ = index.insert_descriptor(0, descriptor); // Panics inside this call.
}
```

Replacing `/0/*'` with `/0'/1` or `/<0;1>/*` reproduces the other cases.

**Expected behavior**

Unsupported descriptor forms should be handled without panicking, allowing callers to handle failed insertion through the API.

Start in the KeychainTxOutIndex::insert_descriptor implementation and inspect its descriptor_id() call before validation. Add or run the regression test in crates/chain/tests/test_keychain_txout_index.rs for the hardened wildcard, alternate derivation, and multipath forms. Done means each unsupported descriptor returns through the Result API without panicking.

Written by the indexing model from the issue text.