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, using idiomatic slice pattern matching (if let [recipient] = ...).

Also fixes a minor grammar issue in the error message ("drain to" → "drained to").

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.
• I used if let [recipient] = recipients.as_slice() instead of recipients.len() == 1 + indexing, as it's more idiomatic Rust and eliminates the possibility of a panic on empty slices.
• The grammar fix ("drain to" → "drained to") is applied in two places where the same typo was duplicated.
• The integration test tests/send_all_validation.rs verifies the fix end-to-end: it sets up a wallet via wallet config, then calls create_tx --send_all with two recipients and asserts that the command fails with the expected error message.

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

New Features:

• I've added tests for the new feature
• I've added docs for the new feature
• 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