Make Response and InvokeAsync non-nullable; unify request/response constraints

[marklauter]

Summary

Fixes a long-standing design flaw: RequestContext.Response was TResponse? and InvokeAsync returned Task<TResponse?>, forcing every caller to null-check a response the pipeline is expected to produce.

• Constrain TResponse : notnull across the core, the static factories, the RequestMiddleware delegate, and PlumberApplicationFactory.
• RequestContext.Response is now a non-nullable TResponse (initialized to default!); InvokeAsync (handler + factory) returns Task<TResponse>.
• Unify the request constraint: the Diagnostics and Serilog extensions used TRequest : class while the core used TRequest : notnull. That blocked value-type requests (e.g. int) from compiling against the diagnostics/logging middleware even though the core pipeline accepts them. Nothing in those bodies relied on reference semantics, so class → notnull everywhere.

Request and response are now symmetric — any non-null value or reference type, with Unit as the "nothing" type on either side.

	Request	Response
Constraint	TRequest : notnull	TResponse : notnull
Allowed	any value/reference type	any value/reference type
"nothing"	Unit	Unit
Disallowed	null	null

Note on runtime semantics

The notnull constraint stops callers supplying a nullable type argument, and the property/return read as non-null. There is no runtime guard forcing middleware to assign Response — a reference-type pipeline that returns without setting it yields default! typed as non-null. This is deliberate: Unit event-style pipelines rely on default(Unit) being the valid sole value, and it matches the ASP.NET HttpContext.Response convention.

Breaking change

Public signatures of RequestContext.Response, InvokeAsync, and the generic constraints on the extension methods change. Major version bump.

Verification

• dotnet build — clean, 0 warnings
• dotnet format --severity info --verify-no-changes — clean
• dotnet test — 179 passed, 0 failed, 100% line/branch/method coverage

🤖 Generated with Claude Code