protocols/rpc: add UseUpgradedAuth flag to SimulateTransactionRequest

[Ryang-21]

What

Adds an AuthV2 UseUpgradedAuth field (json:"useUpgradedAuth,omitempty") to protocols/rpc.SimulateTransactionRequest,
the request type for the RPC simulateTransaction method. When set to true, it asks the
simulator to record authorization entries using SorobanCredentials AddressV2 ("v2")
instead of Address ("v1").

Also adds test coverage:

• JSON (de)serialization of the new field, including omitempty behavior and a full round-trip.
• XDR round-trip tests for the v2 Soroban authorization types these credentials rely on:
  SOROBAN_CREDENTIALS_ADDRESS_V2, SOROBAN_CREDENTIALS_ADDRESS_WITH_DELEGATES,
  SorobanDelegateSignature (including recursive nested delegates), and the
  ENVELOPE_TYPE_SOROBAN_AUTHORIZATION_WITH_ADDRESS HashIDPreimage arm.

Why

Protocol 27 introduced a v2 authorization model with new credential XDR types. The
simulateTransaction RPC method exposes an authV2 useUpgradedAuth request flag so clients can opt into
having recorded auth entries returned in the v2 credential format. Surfacing this on the
SDK's request type lets rpcclient (and other consumers of protocols/rpc) request v2 auth
recording. The XDR round-trip tests lock in correct encode/decode behavior for the newly
generated v2 credential types.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds test coverage and API surface for Soroban auth “v2” constructs and an authV2 flag on the RPC simulate-transaction request.

Changes:

• Introduces XDR round-trip tests for Soroban credential v2 union arms, delegate signatures, and a new hash-id preimage arm.
• Adds authV2 to SimulateTransactionRequest and verifies JSON marshal/unmarshal behavior.
• Extends RPC tests to cover authV2 defaulting and round-tripping.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
xdr/soroban_credentials_test.go	New XDR round-trip and union-arm mapping tests for Soroban auth v2 types.
protocols/rpc/simulate_transaction_test.go	Adds JSON tests for the new authV2 request field.
protocols/rpc/simulate_transaction.go	Adds AuthV2 to SimulateTransactionRequest.

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