Refactor RPC client into a standalone injectable dependency

[Kingsman-99]

Overview

The RPC client is instantiated directly inside StellarSplitSDK, making it impossible to swap in a custom transport for testing or alternative environments without monkey-patching. Refactor it into an injectable dependency.

Acceptance Criteria

• RpcClient interface extracted with methods: simulateTransaction, sendTransaction, getTransaction, getEvents, getLatestLedger
• StellarSplitSDK({ rpcClient: customClient }) constructor option accepts a custom RpcClient implementation
• Default SorobanRpcClient created internally when rpcClient not supplied (backward-compatible)
• MockRpcClient exported from @stellar-split/sdk/testing — implements RpcClient with configurable responses
• All existing connection-pooling, retry, and rate-limiting logic wraps RpcClient calls (not the Soroban SDK directly)
• TypeScript: RpcClient interface exported from the package root
• Unit tests replaced with MockRpcClient — no real network calls in unit test suite after this change

[Turawa2]

@Turawa2 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I can work on the issue perfectly.

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[MorsH14]

@MorsH14 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Id love to work on this issue

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[TheShnider]

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

Hi. I have reviewed this issue and would like to work on it.

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[estyemma]

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

Please assign this issue to me

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[valoryyaa-byte]

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

i would love to work on this

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