Update tx queue memo filtering

[sisuresh]

Description

Resolves #X

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document

[Copilot]

Pull Request Overview

This PR updates the memo filtering logic for Soroban transactions by removing the special case handling for source-account-only authorization. The change simplifies the validation to always reject Soroban transactions that have memos or use muxed ED25519 accounts, regardless of the authorization type.

• Removes the source account authorization check that previously allowed memo usage
• Simplifies the validation logic to a single conditional check
• Updates the corresponding test to expect ADD_STATUS_ERROR instead of ADD_STATUS_PENDING

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/herder/TransactionQueue.cpp	Simplifies validateSorobanMemo function by removing source account auth logic and combining memo/muxed account checks
src/herder/test/TransactionQueueTests.cpp	Updates test expectation from ADD_STATUS_PENDING to ADD_STATUS_ERROR for Soroban tx with memo

[Copilot]

This test change indicates a behavior modification in transaction processing. According to the coding guidelines, fee bump tests should be written to accompany any logic changes to transaction processing. Consider adding fee bump test coverage for this memo filtering change.