Merge bitcoindevkit/bdk-cli#350: refactor(config): remove redundant wallet field...

bc050c71f322d50cfa024a79a8760ce305227add refactor(config): remove redundant wallet field from WalletConfigInner (Jennycj)

Pull request description:

  ### Description

  Removes the redundant `wallet` field from `WalletConfigInner`. The wallet name was being stored twice: once as the key in `WalletConfig::wallets`, and once inside each entry as `wallet: String`. The `wallets` command's JSON/TOML output showed both.
  For example:

  ```json
  {
    "my_wallet": {
      "wallet": "my_wallet",
      "network": "testnet",
      ...
    }
  }
  ```

  The `"wallet"` field inside the entry is gone and the name is still available as the map key.

  Fixes #332

  ### Notes to the reviewers

  - `TryFrom<&WalletConfigInner> for WalletOpts` needed to be removed because it only receives the struct. It is now  replaced with a private `try_to_wallet_opts(&self, wallet_name: &str)` that takes the name from the caller instead.

  - Backward compatible: old config.toml files with the stray `wallet = "..."` line still load (serde ignores unknown fields). This is covered by`test_load_tolerates_legacy_wallet_field`.

  - CHANGELOG updated under `[Unreleased]`.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk-cli/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  tvpeter:
    ACK bc050c71f322d50cfa024a79a8760ce305227add
  vadim-anfv:
    ACK bc050c71f322d50cfa024a79a8760ce305227add; successfully ran local tests; thanks!

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