Do not bypass simulation result auth entries processing

[aditya1702]

What

• Process the simulation result's Results array when building transaction.
• Add a check for the case where operation is InvokeHostFunction and the simulationResult.Results field is empty.

Why

TODO

Known limitations

N/A

Issue that this PR addresses

Closes #502

[Copilot]

Pull request overview

This PR tightens Soroban transaction handling to prevent bypassing auth-entry verification when building/signing channel-account transactions, and extends the GraphQL simulation-result input conversion to carry through host function results.

Changes:

• Reject Soroban InvokeHostFunction transactions when the simulation response has empty/nil results, ensuring auth entries cannot be skipped.
• Extend GraphQL convertSimulationResult to parse and populate simulation results from JSON-encoded strings.
• Add/extend unit tests covering the new validation and GraphQL conversion behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
internal/services/transaction_service.go	Adds a new validation error and enforces non-empty simulation Results for InvokeHostFunction.
internal/services/transaction_service_test.go	Adds test cases to confirm InvokeHostFunction rejects empty/nil results while other Soroban ops may allow it.
internal/serve/graphql/resolvers/resolver.go	Parses SimulationResultInput.Results strings into entities.RPCSimulateHostFunctionResult.
internal/serve/graphql/resolvers/mutations.resolvers.go	Maps the new Soroban validation error to INVALID_SOROBAN_TRANSACTION.
internal/serve/graphql/resolvers/mutations_resolvers_test.go	Adds tests for GraphQL error mapping and for convertSimulationResult results parsing.

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