Add unit tests for the Stellar memo validator accepting only text memos within the 28-byte limit

[Chucks1093]

Summary

Stellar text memos are limited to 28 bytes. The memo validator used before transaction submission should reject memos that exceed the byte limit, are empty, or contain non-UTF-8 characters, and accept valid memos at the boundary.

Scope

• Test a memo of exactly 28 ASCII characters passes validation
• Test a memo of 29 ASCII characters fails with memo_too_long
• Test a memo of 28 UTF-8 multi-byte characters that exceed 28 bytes fails with memo_too_long
• Test an empty string memo fails with memo_empty
• Test a valid short memo passes unchanged

Acceptance Criteria

• 28-byte memo passes
• 29-byte memo fails with memo_too_long
• Multi-byte characters counted by byte length, not character count
• Empty memo fails with memo_empty
• Valid short memo passes unchanged

ETA: 12 hours

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