Merge bitcoindevkit/bdk-cli#291: fix(create_tx): enforce single recipient for send_all

Merge bitcoindevkit/bdk-cli#291: fix(create_tx): enforce single recipient for send_all

50d248cfc482bf5d7807b84182e7f19b104ab49b fix(create_tx): enforce single recipient for send_all (Artemiy)

Pull request description:

  Closes #290

  ### Description

  Previously, passing multiple `--to` arguments with `--send_all` would silently drain the wallet to only the first recipient, ignoring the rest. This was inconsistent with the silent-payments `create_tx`, which already validates the recipient count.

  Now returns an error unless exactly one recipient is provided.

  ### Notes to the reviewers

  - The fix mirrors the validation logic already present in the silent-payments `create_tx` branch of the same file, so behavior is now consistent between the two code paths.

  ## Changelog notice
  - Fixed create_tx --send_all to reject multiple recipients instead of silently using only the first one

  ### 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

  * [x] I've updated `CHANGELOG.md`

  #### Bugfixes:

  * [ ] This pull request breaks the existing API
  * [ ] I've added tests to reproduce the issue which are now passing
  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  tvpeter:
    ACK 50d248cfc482bf5d7807b84182e7f19b104ab49b
  va-an:
    ACK 50d248cfc482bf5d7807b84182e7f19b104ab49b

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