fix(create_tx): enforce single recipient for send_all

[1estart]

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:

• I've signed all my commits

• I followed the contribution guidelines

• I ran cargo fmt and cargo clippy before committing

• 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
• I'm linking the issue being fixed by this PR

[va-an]

The handlers.rs fix looks good.

As discussed in #225 (comment) we're moving away from this style of testing (driving the binary as a subprocess), so I'd suggest not adding tests in this PR and dropping the new test file. We can add proper coverage as a follow up after #278 and #289 are merged.

[1estart]

The handlers.rs fix looks good.

As discussed in #225 (comment) we're moving away from this style of testing (driving the binary as a subprocess), so I'd suggest not adding tests in this PR and dropping the new test file. We can add proper coverage as a follow up after #278 and #289 are merged.

Understood. I've kept the changes minimal and dropped the test file as suggested. I can add the tests later once the related PRs are merged.