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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Coordinate on Telegram

[LumDev86]

Hi — I'd like to take this one.

TypeScript with Jest is my daily testing setup. The interesting case here
is the third one: 'á'.length is 1 in JavaScript but 2 bytes in UTF-8, so a
28-character memo can be well over the 28-byte limit. I'd assert against
Buffer.byteLength(memo, 'utf8') rather than .length, so the test actually
proves byte counting and would fail if someone swapped the implementation
back to character counting.

Plan — the five scope cases as separate Jest cases:

• 28 ASCII characters → passes
• 29 ASCII characters → fails with memo_too_long
• 28 multi-byte UTF-8 characters (over 28 bytes) → fails with memo_too_long
• empty string → fails with memo_empty
• valid short memo → passes unchanged

I'll follow the existing test file layout and run pnpm test before opening
the PR. I saw the 12-hour ETA and can meet it.

Could you assign it to me? Happy to coordinate on Telegram.