.NET: Fix misleading workflow protocol attribute diagnostics

[Peter Ibekwe (peibekwe)]

Motivation & Context

Protocol-only executors incorrectly receive diagnostics referring to [MessageHandler]. This makes the intentional partial requirement confusing.

Description & Review Guide

• What are the major changes? Add dedicated diagnostics for  [SendsMessage]  and  [YieldsOutput] , clarify documentation, and add regression tests.
• What is the impact of these changes? Users receive accurate, actionable errors without changing existing behavior.
• What do you want reviewers to focus on? Diagnostic wording and separation from handler diagnostics.

Fixes #5163

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

Pull request overview

Adds protocol-specific diagnostics so executors using [SendsMessage] or [YieldsOutput] receive accurate guidance.

Changes:

• Adds MAFGENWF008 and MAFGENWF009.
• Routes protocol-only validation through the new diagnostics.
• Documents and tests the partial requirement.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
ExecutorRouteGeneratorTests.cs	Adds regression coverage for protocol diagnostics.
YieldsOutputAttribute.cs	Documents the partial requirement.
SendsMessageAttribute.cs	Documents the partial requirement.
DiagnosticDescriptors.cs	Defines protocol-specific diagnostics.
SemanticAnalyzer.cs	Uses the new diagnostics for protocol-only classes.

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[github-actions]

Agent Framework Review — Iteration 1

Completed passes: 5 | Result: No high-severity findings

Scope: full PR (1 commit(s)): dec11d76b826

Review passes

• Correctness (gpt-5.6-sol) — No issues found in this pass.
• Security Reliability (claude-opus-4.8) — No issues found in this pass.
• Test Coverage (gpt-5.6-sol) — No issues found in this pass.
• Failure Modes (claude-opus-4.8) — No issues found in this pass.
• Design Approach (claude-opus-4.8) — No issues found in this pass.