Add CLI command to preview generated code for a specific handler or HTTP endpoint

[jeremydmiller]

Summary

Add a CLI command that outputs the generated source code for a single message handler or HTTP endpoint, rather than dumping all generated code via codegen preview.

Proposed usage

# By message type name
dotnet run -- codegen preview --handler MyApp.Commands.CreateOrder

# By HTTP route
dotnet run -- codegen preview --route "POST /api/orders"

# By handler class name
dotnet run -- codegen preview --handler CreateOrderHandler

Motivation

dotnet run -- codegen preview dumps the generated code for every handler and endpoint, which can be thousands of lines in a real application. When troubleshooting a specific handler — checking middleware ordering, service resolution, transaction wrapping, or exception handling — you need to find the relevant section in a wall of output.

A targeted command would:

1. Make troubleshooting faster — go directly to the handler you care about
2. Enable AI-assisted tooling — coding agents can request the generated code for a specific handler to diagnose issues (middleware not applied, service location instead of inlining, missing transaction wrapping)
3. Support dotnet run -- describe-style quick diagnostics without code changes

Output should include

• The full generated handler adapter class
• Which middleware methods are woven in and their order
• How each dependency is resolved (inlined singleton, generated constructor call, or service location)
• Transaction wrapping if present

Context

The AI skills for Critter Stack plan to use this for troubleshooting Wolverine behavior in AI-assisted workflows. Being able to ask "show me the generated code for this specific handler" is a key diagnostic capability.