[SDK] Implement mock RPC server for SDK unit tests

[Utilitycoder]

Create a reusable mock server for stellar-sdk RPC calls to avoid network dependency in unit tests. This will make the test suite faster and more reliable.

[Obiajulu-gif]

@Obiajulu-gif has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

My Cheif, can I work on this issue

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[mandyslovestories-sudo]

@mandyslovestories-sudo has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

I'd love to work on this issue

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[drips-wave]

Congratulations, @mandyslovestories-sudo! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

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