Add fuzz testing for EphemeralAccount initialize() parameters

[phertyameen]

Issue #57: Add fuzz testing for EphemeralAccount initialize() parameters

Category: Testing
Repo: bridgelet-core (Soroban / Rust)

Description

Use cargo-fuzz or afl to fuzz the initialize() function with random: expiry_ledger values, amounts, and address formats. Target any panics or unexpected error codes.

Acceptance Criteria

• Rust implementation complete
• cargo test passing
• Integration test on local Stellar sandbox passing
• Docs updated (docs/ or inline rustdoc)
• Reviewed and merged