Add defensive coding and matching unit tests for fee bumps

[Shaptic]

What

Add guardrails around accessing possibly-nil pointers when inspecting transactions.

Why

When passing a fee-bump transaction containing a soroban operation, the following check:

			if txEnvelope.Type != xdr.EnvelopeTypeEnvelopeTypeTx && txEnvelope.V1.Tx.Ext.V != 1 {

would cause a nil pointer reference because V1 is nil. While this has no security implications (the panic is simply caught and logged like any other error), it is good to be defensive.

[Copilot]

Pull request overview

This PR adds defensive nil pointer checks when extracting Soroban transaction data from transaction envelopes, preventing panics when processing fee-bump transactions containing Soroban operations. The changes refactor the code into well-structured helper functions and add comprehensive test coverage.

Changes:

• Introduced sorobanDataFromEnvelope and sorobanDataFromTx helper functions with defensive nil checks for all pointer accesses
• Refactored existing code to use new helper functions, eliminating unsafe nil pointer access in fee-bump transaction handling
• Added unit test verifying that fee-bump transactions with missing Soroban data return appropriate error messages instead of panicking

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
cmd/stellar-rpc/internal/methods/simulate_transaction.go	Added two helper functions with comprehensive nil checks; replaced unsafe direct field access with calls to these helpers
cmd/stellar-rpc/internal/methods/simulate_transaction_test.go	Added test case and helper function to verify fee-bump transactions without Soroban data are handled gracefully

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