Add unit test for simulate_transaction with failed simulation response

[Emrys02]

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Simulation failures (e.g., contract traps, budget exceeded) return a specific JSON structure. We need a test to ensure simulate_transaction correctly parses these errors and returns the appropriate PrismError::RpcError.

Implementation Guidelines Key Files: crates/core/src/rpc/client.rs.
Expectations What done looks like: A unit test verifies that simulate_transaction returns an error when the RPC response contains a simulation failure, with the error message correctly extracted.

For any question, reach me on Discord: emry_ss

[zeemscript]

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

hi, please assign me this issue

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[drips-wave]

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

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