middleware: build Maybe handler once

[lowgame]

Reproduction

middleware.Maybe called the middleware constructor once per matching request. A stateful constructor therefore reported two builds after serving two requests through the same handler.

The added TestMaybeBuildsMiddlewareOnce fails on the previous implementation with:

middleware was built 2 times, want 1

Root cause

mw(next) was evaluated inside the request handler instead of when the middleware chain was assembled.

Solution

Build the wrapped handler once in Maybe's outer closure, then use that handler whenever the request predicate matches. The predicate is still evaluated independently for every request, and nonmatching requests still go directly to next.

Tests

• GOTOOLCHAIN=go1.25.1 go test ./middleware -run '^TestMaybeBuildsMiddlewareOnce$' -count=20
• GOTOOLCHAIN=go1.25.1 go test -race ./middleware
• GOTOOLCHAIN=go1.25.1 go test -race .
• GOTOOLCHAIN=go1.25.1 go vet ./...
• git diff --check