feat(xpp/http): httptest — three-layer HTTP testing (tower/axum + wiremock aligned)

[mivinci]

Summary

Replaces the 540-line hand-rolled test_server.h with a three-layer testing model aligned with the Rust ecosystem (tower/axum + wiremock — the design discussion is in the deleted todos/httptest.md, now implemented).

Three layers

Layer	What	Status
0	router(req) direct call — handler/router/middleware unit tests (in-process, no sockets)	✅ already existed (PR #85's Router-as-Handler)
1	test::Server — real TCP fixture for client tests & end-to-end	this PR
2	test::EvilServer — fault injection (truncated Content-Length)	this PR

test::Server (http/test/server.h)

// Handler-driven (any behavior — the wiremock "arbitrary" form):
test::Server ts([](Request req) { return Response::ok("hello"); });
auto r = client.get(ts.url().c_str()).await();

// Multi-route (Router capabilities: :params, layers, async handlers):
test::Server routed([](ServerBuilder &b) {
  b.route("GET /users/:id", [](Request req, String id) { return Response::ok(id); });
});

// Data-driven (preset spec):
TestResponseSpec spec;
spec.body = Bytes::from("hello");
test::Server ts2(spec);

Three constructors dispatched by first-arg type (first_arg trait). Construct = start (bind 127.0.0.1:0 + listen sync — port() valid immediately). RAII dtor = stop + drain. Bind failure asserts (Go httptest.NewServer panic semantics).

The spec compiles into Router routes — no hand-rolled HTTP anywhere. All seven fields work: static/echo-body/echo-method/redirect/delay/mid-body-stall/headers. (Mid-body stall = channel body with delayed producer: pausing production pauses transmission.)

test::EvilServer (http/test/evil_server.h)

Raw TCP (~100 lines): declares full Content-Length, sends partial body, hard-closes — the mid-body-disconnect fault that a well-formed server cannot produce (wiremock's Fault::Abort family). Migrated from the old spec's truncate_body_after.

Per-file tests (the "each file gets a test" convention)

• http/test/server_test.cpp — 7 tests: handler ctor, spec ctor, redirect, echo body, echo method, builder-configure (:param injection), delay
• http/test/evil_server_test.cpp — 1 test: declared 1MB / sent 64KB truncation surfaces as a body-read error

Migration

• client_test.cpp (13 tests) + http_convenience_test.cpp (2 tests): old TestServer → test::Server; truncation test → test::EvilServer
• Old http/test_server.h deleted (−540 lines of hand-rolled HTTP parsing, response building, reason-phrase table, Connection: close management)

Bug found & fixed during migration

The spec's echo_request_method didn't work alongside echo_request_body — the method was consumed with the Request before it could be captured. Fixed: capture the method before req.into_body().

Verification

• ASan full suite: 83/83 (69 existing zero-breakage + 14 new)
• All existing client/convenience tests pass unchanged against the new fixture
• clang-format clean