Fix issues 394, 395, 396, 397: Core SDK Validation, Health, and Cache

[wowamaxe-glitch]

This Pull Request implements fixes and enhancements for several core functionalities in the SDK:

1. Address Validation Utilities (Closes Add Stellar address validation and formatting utility functions #394):

   • Implemented isValidStellarAddress using StrKey.isValidEd25519PublicKey.
   • Added addressesEqual, toMuxedAddress and fromMuxedAddress.

2. Client Health Check (Closes Add SDK health check method with RPC connectivity report #395):

   • Added healthCheck() method to StellarSplitClient.
   • Uses getContractWasmByContractId to verify contract existence.
   • Enforces a 5-second timeout with HealthCheckTimeoutError.

3. Configuration Validation on Instantiation (Closes Validate SDK configuration at instantiation and surface clear errors #396):

   • Renamed generic error to InvalidConfigError.
   • SDK client now throws InvalidConfigError synchronously during instantiation.

4. Simple LRU Cache Eviction Policy (Closes Implement LRU eviction policy for the SDK in-memory cache #397):

   • Upgraded SimpleCache to use an LRU eviction policy.
   • Cache automatically ejects least recently used entries when maxEntries limit is exceeded.

Unit tests were added and updated.

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