feat: Add account sponsorship, per-endpoint rate limiter, and client config validation (#213, #214, #137)

[sublime247]

Summary

Closes #213
Closes #214
Closes #137
This PR implements the solution for issues #213, #214, and #137:

1. Account Sponsorship Utilities (#213)

• Added setSponsor(account, sponsor) and removeSponsor(account) functions in src/account/sponsorship.ts.
• Validates account and sponsor public key format using isValidPublicKey.
• Prevents self-sponsorship.
• Constructs operation sequence (beginSponsoringFutureReserves + endSponsoringFutureReserves or revokeAccountSponsorship) and tracks required signers.
• Exposed methods on client.account and public SDK entry points.

2. Advanced Rate Limiter with Per-Endpoint Quotas (#214)

• Extended TokenBucketRateLimiter in src/shared/utils.ts to support per-endpoint quotas.
• Provided sensible default limits per endpoint category (contract.simulate: 5 req/s, account.get: 20 req/s, etc.).
• Added setEndpointLimit() for runtime overrides and handleResponseHeaders() for X-Rate-Limit-* header parsing.

3. Client Configuration Validation on Startup (#137)

• Added validateClientConfig() in src/client/createSorokitClient.ts.
• Checks required network parameter, well-formed URLs (horizonUrl, rpcUrl), and interface methods on cache and logger.
• Returns detailed validation errors with typed SorokitErrorCode.INVALID_CONFIG or INVALID_NETWORK.

Verification

• Added comprehensive unit tests in sponsorship.test.ts, client.test.ts, and shared.test.ts.
• Verified clean build via npm run build and tests via npm test.

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