Add wolverine-diagnostics codegen-preview CLI command

[jeremydmiller]

Summary

Closes #2467.

• Adds WolverineDiagnosticsCommand (wolverine-diagnostics) to src/Wolverine/Diagnostics/ as an extensible parent command using a string-typed sub-command action (JasperFx/Oakton's idiomatic pattern — no true sub-command mechanism exists; enum-style dispatch used via string switch).
• First sub-command is codegen-preview with --handler and --route flags.
• Command lives in core Wolverine package; HTTP route support avoids a direct compile-time dependency on Wolverine.Http by matching against ICodeFile.FileName (mirrors HttpChain.determineFileName() logic).

Usage:
```bash

By fully-qualified message type

dotnet run -- wolverine-diagnostics codegen-preview --handler MyApp.Orders.CreateOrder

By short class name or handler class name (fuzzy match)

dotnet run -- wolverine-diagnostics codegen-preview --handler CreateOrderHandler

HTTP endpoint

dotnet run -- wolverine-diagnostics codegen-preview --route "POST /api/orders"
dotnet run -- wolverine-diagnostics codegen-preview --route "GET /api/orders/{id}"
```

Note on sub-command naming: JasperFx's `Enum.Parse`-based argument conversion does not support hyphens in enum value names. Using a `string` action property allows `codegen-preview` (with hyphen) to work exactly as described in the issue. Future sub-commands (`describe-routing`, `describe-resiliency`) follow the same pattern.

Test plan

• 13 unit/smoke tests in `CoreTests.Diagnostics.WolverineDiagnosticsCommandTests`
  • `RouteInputToFileName` theory covering 8 route patterns including hyphens, route params, no-method prefix
  • `FindHandlerChain` — exact full name, short name, handler class name, fuzzy contains, no-match
  • End-to-end `codegen_preview_generates_code_for_handler` verifying generated C# contains handler name
• Run `dotnet test --filter CoreTests.Diagnostics` — all pass
• Build `Wolverine.csproj` — succeeds with no new errors

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