TASK-069: remove transitional two-arg http_request_impl ctor

The two-arg http_request_impl(MHD_Connection*, unescaper_ptr) overload
was preserved during the M3 PIMPL split (TASK-015 / TASK-016) "for
source compatibility" with callers not yet ported to the allocator-
taking form. The v2 cutover is complete and detail/ headers do not
require a deprecation cycle, so the overload comes out now.

Changes:

- src/httpserver/detail/http_request_impl.hpp: delete the two-arg
  delegating constructor and its comment block. The default ctor
  (test-request path) and the canonical three-arg allocator-aware
  ctor remain.

- src/http_request.cpp: migrate the sole live caller (the
  heap-fallback branch when pick_resource returns the default
  resource) to pass std::pmr::polymorphic_allocator<>(get_default_
  resource()) explicitly. Semantically identical to what the
  delegating ctor did internally; byte-for-byte heap behaviour is
  unchanged.

- test/unit/http_request_pimpl_test.cpp: add three static_asserts
  pinning the construction surface:
    1. default-constructible (test-request path);
    2. canonical three-arg (MHD_Connection*, unescaper_ptr, alloc)
       remains constructible;
    3. negative: !is_constructible from (MHD_Connection*,
       unescaper_ptr) -- the TASK-069 sentinel. If the two-arg
       overload ever returns, the build breaks here.

Verification:

- `make check -j1` (release): 98/98 pass.
- `make check -j1` (--enable-debug, -Werror -Wextra): 98/98 pass.
- Header-only cross-flag sweep compiles the sentinel TU under each
  of -UHAVE_BAUTH / -UHAVE_DAUTH / -UHAVE_GNUTLS / -UHAVE_WEBSOCKET
  cleanly (proves the canonical ctor's #ifdef-conditioned member-
  init list still works under every flag-off permutation, per the
  plan's Step B).

Pre-existing on feature/v2.0, NOT introduced by this task:
- check-doxygen.sh fails with 7 warnings (4 @security + start()
  refs + stale arguments_accumulator refs) in
  create_webserver.hpp / hook_context.hpp / webserver_websocket.hpp.
  None of those files are in this diff; tracked separately for a
  future cleanup.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: etr

src/http_request.cpp

@@ -137,7 +137,9 @@ http_request::http_request(struct MHD_Connection* underlying_connection, unescap
     if (res == std::pmr::get_default_resource()) {
         // Heap-fallback: matches v1 lifetime exactly; deleter frees via
         // operator delete.
-        impl_.reset(new detail::http_request_impl(underlying_connection, unescaper));
+        impl_.reset(new detail::http_request_impl(
+            underlying_connection, unescaper,
+            std::pmr::polymorphic_allocator<>(std::pmr::get_default_resource())));
         impl_.get_deleter().fn = &detail::delete_impl_heap;
     } else {
         // Arena-backed: allocate and construct via polymorphic_allocator

src/httpserver/detail/http_request_impl.hpp

@@ -91,14 +91,6 @@ class http_request_impl {
                             std::pmr::polymorphic_allocator<>{
                                 std::pmr::get_default_resource()}) {}
 
-    // Two-arg ctor (TASK-015 surface) is preserved for source compatibility
-    // with any caller that hasn't been ported to the allocator-taking ctor
-    // yet -- it forwards to the three-arg form with the default resource.
-    http_request_impl(MHD_Connection* connection, unescaper_ptr unescaper)
-        : http_request_impl(connection, unescaper,
-                            std::pmr::polymorphic_allocator<>{
-                                std::pmr::get_default_resource()}) {}
-
     // TASK-016: allocator-aware constructor. The PMR-aware containers in
     // this impl propagate `alloc` through their value_types via the
     // standard scoped-allocator semantics built into

test/unit/http_request_pimpl_test.cpp

@@ -31,8 +31,10 @@
 
 // HTTPSERVER_COMPILATION is supplied by test/Makefile.am AM_CPPFLAGS.
 #include "httpserver/http_request.hpp"
+#include "httpserver/detail/http_request_impl.hpp"
 
 #include <map>
+#include <memory_resource>
 #include <string>
 #include <type_traits>
 #include <utility>
@@ -132,4 +134,22 @@ static_assert(std::is_same_v<
                                           httpserver::http::file_info>>&>,
               "get_files must return const std::map<...>&");
 
+// TASK-069: pin the http_request_impl construction surface. After v2
+// cutover, the only public constructors are the no-arg default
+// (test-request path, delegates to the three-arg form) and the
+// allocator-aware three-arg form. The transitional two-arg overload
+// (MHD_Connection*, unescaper_ptr) is gone. If it ever returns, the
+// negative is_constructible assertion below breaks the build.
+static_assert(std::is_constructible_v<httpserver::detail::http_request_impl>,
+              "default http_request_impl() must remain (test-request path)");
+static_assert(std::is_constructible_v<
+                  httpserver::detail::http_request_impl,
+                  MHD_Connection*, httpserver::unescaper_ptr,
+                  std::pmr::polymorphic_allocator<>>,
+              "canonical three-arg http_request_impl(MHD_Connection*, unescaper_ptr, alloc) must exist");
+static_assert(!std::is_constructible_v<
+                  httpserver::detail::http_request_impl,
+                  MHD_Connection*, httpserver::unescaper_ptr>,
+              "TASK-069: transitional two-arg http_request_impl ctor must be removed");
+
 int main() { return 0; }