Add Stellar keypair generation utility

[therealjhay]

Description: The codebase generates IDs using crypto.randomUUID() but never generates Stellar keypairs. For testing and merchant onboarding, keypair generation is needed.

Requirements:

• Create generateStellarKeypair(): { publicKey: string; secretKey: string }
• Use @stellar/stellar-sdk Keypair utilities
• Never log the secret key
• Add a warning JSDoc about secret key security
• Add unit tests verifying the generated keys are valid

Suggested execution steps:

1. Import Keypair from @stellar/stellar-sdk
2. Generate keypair with Keypair.random()
3. Return { publicKey: keypair.publicKey(), secretKey: keypair.secret() }
4. Add JSDoc warning: "The secret key must be stored securely and never logged"
5. Add tests: verify public key passes StrKey.isValidEd25519PublicKey, secret passes StrKey.isValidEd25519SecretSeed

Example commit message:

feat(stellar-utils): add Stellar keypair generation utility

generateStellarKeypair creates random Ed25519 keypairs for testing and
merchant onboarding, with security warnings in documentation.

[TheBigWealth89]

@TheBigWealth89 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi, I'd like to work on this issue! because I am very conversant with solving problems like this

ℹ️ Repo Maintainers: To accept this application, review their application or assign @TheBigWealth89 to this issue.

[charlesejemai-cyber]

@charlesejemai-cyber has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

hi i would like to work on this, thanks

ℹ️ Repo Maintainers: To accept this application, review their application or assign @charlesejemai-cyber to this issue.

[ItunuOni]

@ItunuOni has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would like to work on this

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ItunuOni to this issue.

[drips-wave]

Congratulations, @ItunuOni! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @ItunuOni: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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