Summarize changes in PR #7439: Explicit AddHandler registration API

[Copilot]

This PR introduces an explicit handler registration API for NServiceBus, replacing automatic handler discovery with compile-time verified, developer-controlled registration.

Summary of Changes

Core API Changes

• New explicit registration: config.AddHandler<THandler>() replaces automatic scanning
• Handler execution order: Determined by registration order, replacing ExecuteTheseHandlersFirst()
• Breaking change: ExecuteTheseHandlersFirst() marked obsolete (error-level)

Architecture

• Source generator: AddHandlerInterceptor generates optimized registration code at compile time
• Type-safe invocation: Generic MessageHandlerInvoker<THandler, TMessage> replaces reflection-based dispatch
• Registry enhancements: Strongly-typed registration methods with deduplication
  • AddMessageHandlerForMessage<THandler, TMessage>()
  • AddTimeoutHandlerForMessage<THandler, TMessage>()

Analyzer (NSB0020)

• Prevents injection of IMessageSession/IEndpointInstance into handlers
• Enforces use of IMessageHandlerContext parameter

Message Type Discovery

• Separated handler scanning from message type discovery
• Message types now registered from both conventions and handler interfaces
• Handles message hierarchy correctly when base types not in EnclosedMessageTypes header

Migration Example

Before:

config.ExecuteTheseHandlersFirst(typeof(FirstHandler), typeof(SecondHandler));

After:

config.AddHandler<FirstHandler>();
config.AddHandler<SecondHandler>();

Impact

• Compile-time handler verification
• Explicit control over handler order and registration
• Performance improvements via source-generated registration
• Breaking change requiring code migration

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PR Stats: 73 commits, 66 files, +1,481/-927 lines
Target: vNext (v11.0.0+)

Original prompt

Summarize the changes in #7439

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