Add `# Errors` section to `assert_unique_recipients` and `assert_recipients_have_trustlines`

[Kingsman-99]

Description

assert_unique_recipients and assert_recipients_have_trustlines in contracts/split/src/validation.rs lack # Errors doc sections. Without them, callers cannot tell from docs alone what error variant is returned in the failure path.

Acceptance Criteria

• Add /// # Errors\n/// Returns [ContractError::DuplicateRecipient] when a duplicate is found. to assert_unique_recipients
• Add /// # Errors\n/// Returns [ContractError::RecipientMissingTrustline] when a balance call fails. to assert_recipients_have_trustlines
• cargo doc --no-deps succeeds without warnings
• cargo test passes

Context

• Target file: contracts/split/src/validation.rs