pandaproxy: register default_404_handler in server::start()

Wires the new handler so Seastar's routes table dispatches to it for
any (method, path) combination that no registered route matches.
Without this, Seastar's built-in routes::handle() short-circuits with
its own JSON body that uses {"code": 404} instead of the
{"error_code": 404} shape SR clients expect, breaking 404-fallback
paths in external SR client serializers.

The handler is owned by pandaproxy::server via unique_ptr because
Seastar's add_default_handler does not take ownership (per
seastar/include/seastar/http/routes.hh:136). Declared before _server
so it is destroyed after, giving the strongest lifetime guarantee for
the routes table's raw pointer to it.

Both REST proxy and schema registry use pandaproxy::server, so this
fixes the body shape for both subsystems with a single registration.

Author: nguyen-andrew

src/v/pandaproxy/server.cc

@@ -12,6 +12,7 @@
 #include "model/metadata.h"
 #include "net/dns.h"
 #include "net/tls_certificate_probe.h"
+#include "pandaproxy/default_404_handler.h"
 #include "pandaproxy/json/types.h"
 #include "pandaproxy/logger.h"
 #include "pandaproxy/probe.h"
@@ -258,6 +259,10 @@ ss::future<> server::start(
     _server._routes.register_exeption_handler(
       exception_replier{ss::sstring{name(_exceptional_mime_type)}, _log});
 
+    _default_handler = std::make_unique<default_404_handler>(
+      ss::sstring{name(_exceptional_mime_type)});
+    _server._routes.add_default_handler(_default_handler.get());
+
     _probe = std::make_unique<server_probe>(_ctx, _public_metrics_group_name);
 
     _ctx.advertised_listeners.reserve(endpoints.size());

src/v/pandaproxy/server.h

@@ -136,6 +136,7 @@ class server {
     ss::future<> stop();
 
 private:
+    std::unique_ptr<ss::httpd::handler_base> _default_handler;
     ss::httpd::http_server _server;
     ss::sstring _public_metrics_group_name;
     ss::gate _pending_reqs;