Fix: replace string manipulation with stronger Bitcoin types in Caravan export

This commit addresses @ValuedMammal  feedback by replacing extensive string
manipulation with proper Bitcoin types throughout the Caravan wallet export
functionality, making the code more type-safe and less error-prone.

Key Changes:

- Type-safe BIP32 handling: Updated CaravanExtendedPublicKey to use
   bitcoin::bip32::{DerivationPath, Fingerprint, Xpub} instead of strings.
   Added custom serde implementation that properly handles "m/" prefix for
   JSON compatibility while using proper types internally.

- Enhanced descriptor parsing: Changed extract_xpubs_from_multi() to
   accept SortedMultiVec<DescriptorPublicKey, Ctx> instead of
   SortedMultiVec<String, Ctx>, eliminating string parsing errors and
   making the code more robust.

- Enum-based address types: Replaced string-based address type handling
   with CaravanAddressType enum (P2SH, P2WSH, P2SHWrappedP2WSH) for better
   type safety and validation.

- Proper descriptor construction: Updated to_descriptors() to use
   Descriptor::new_sh_sortedmulti(), new_wsh_sortedmulti(), and
   new_sh_wsh_sortedmulti() construction methods instead of string building,
   ensuring standard format with checksums.

- Network type conversion: Replaced string-based network handling with
   CaravanNetwork enum and From<bitcoin::Network> implementation using
   NetworkKind pattern.

- Comprehensive validation: Enhanced test suite to verify that exported
   descriptors can create functional BDK wallets with proper address
   generation, ensuring practical usability.

Author: vkprogrammer-001