[DANTE-1903]

fix(transaction): reject empty and whitespace-only memos by default (#846)
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### Summary

Closes #846                                                                                      
                                                                                                 
This PR adds validation to reject empty and whitespace-only memos by default across              

TransactionBuilder, transaction builder functions, and compose(), while allowing callers to
explicitly opt-in to empty or whitespace memos when needed.

### Changes Made                                                                                 
                                                                                                 
- **TransactionBuilder**:                                                                        
  - Implemented `setMemo(memo: string, options?: SetMemoOptions)` and `getMemo()`.               
  - Added default validation to throw an error (`"Memo cannot be empty or whitespace-only"`) when

passed an empty or whitespace-only string, with { allowEmpty: true } opt-in support.
- Memo Policy Enforcement:
- Updated validateMemoPolicy() in src/transaction/buildTransaction.ts to reject empty or
whitespace-only memos by default with TX_BUILD_FAILED ("Memo cannot be empty or whitespace- only").
- Supported opt-in via allowEmptyMemo?: boolean or memoValidation: { allowEmpty: true }.
- Preserved backward compatibility for rule: "prohibit" where omitting or passing an empty
memo string represents no attached memo.
- Builders & Compose Integration:
- Updated resolveMemo in buildHelpers.ts and transactionContext.ts to respect
allowEmpty configurations.
- Added allowEmptyMemo?: boolean to ComposeOptions and { allowEmpty?: boolean } options
parameter to compose().addMemo().
- Types & Exports:
- Added allowEmpty?: boolean to MemoValidationConfig.
- Added allowEmptyMemo?: boolean to MemoParams.
- Exported SetMemoOptions from src/transaction/index.ts.
- Tests:
- Added comprehensive test coverage in src/tests/memo-validation-length.test.ts testing
createTransactionBuilder().setMemo(), validateMemoPolicy(), and compose().addMemo() edge
cases (zero-length strings, whitespace characters \t\n , and opt-ins).
- Updated existing builder tests in src/tests/transaction.test.ts.

### Verification

All checks have passed locally:
- `npm run typecheck`
- `npm run lint`
- `npm test` (3862 passing tests)
- `npm run build`

closes #846

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