Fix: Add missing error handling for Keypair.fromSecret() in API

[Smartdevs17]

Problem

In api/src/services/stellar.service.ts, Keypair.fromSecret(userSecret) is called inside try-catch blocks, but the error message doesn't distinguish between invalid key format vs other errors.

Context

If a user provides an invalid secret key, the error message is generic. Users need to know specifically that their key format is wrong.

Proposed Solution

Catch the specific error from Keypair.fromSecret():

try {
  const keypair = Keypair.fromSecret(userSecret);
} catch (error) {
  if (error instanceof Error && error.message.includes('Invalid')) {
    throw new ApiError(400, 'Invalid secret key format');
  }
  throw error;
}

Acceptance Criteria

• Invalid secret key returns 400 with specific message
• Valid but wrong key proceeds normally
• Error message does NOT echo the key value
• Tests for invalid key scenarios

Technical Notes

• File: api/src/services/stellar.service.ts

Constraints

• Never include the key value in error responses or logs

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[OperaCode]

I’ve reviewed "api/src/services/stellar.service.ts" and searched across the repository, but I couldn’t find any usage of "Keypair.fromSecret". the current implementation only works with public addresses and does not handle secret keys.

Could you please confirm:

• if this logic has been removed

Happy to proceed once clarified