Memo filter 2

[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 simplifies the memo validation logic for Soroban transactions by removing the source account authentication check. The change makes all Soroban transactions with memos return an error, regardless of whether they use source account authentication only.

• Removes complex source account authentication validation logic from validateSorobanMemo
• Consolidates memo and muxed account validation into a single condition
• Updates test expectation to reflect stricter memo validation

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/herder/TransactionQueue.cpp	Simplifies validateSorobanMemo function by removing source account auth checks and consolidating validation logic
src/herder/test/TransactionQueueTests.cpp	Updates test expectation from ADD_STATUS_PENDING to ADD_STATUS_ERROR for Soroban tx with memo
.github/copilot-instructions.md	Adds GitHub Copilot instructions for fee bump transaction test coverage

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