[Server][Tests] Serve the modern (SEP-2575) lifecycle (#444)

* [Server][Tests] Serve the modern (SEP-2575) lifecycle

The revision removed the initialize handshake: every request carries its own
_meta - protocol version, client capabilities, client info - and answers
server/discover for what initialize used to return. There is no session to
keep, so there is nothing for the handshake-era dispatcher to do.

Builder::buildStateless() produces StatelessProtocol, and
StatelessHttpTransport serves it over one POST-only endpoint - GET and
DELETE are 405, and no Mcp-Session-Id exists to hand out.

Results go through a per-era wire codec rather than the handlers, so
resultType and the caching hints are stamped for 2026-07-28 and left off the
revisions that do not define them.

Covered by the modern conformance suite (make conformance-draft-server)
against its own baseline; unit coverage arrives with the fixes stacked on
this.

* Apply suggestions from code review

Co-authored-by: Christopher Hertel <mail@christopher-hertel.de>

* Fix critical stateless-lifecycle bugs from review

- Don't leak internal exception messages to clients on uncaught
  \Throwable; use a generic message like the handshake-era protocol
  and every built-in handler already do.
- Default a missing/invalid request id to null instead of "", and
  stop answering notifications and malformed messages as if they
  were "method not found" 404s.
- Read the POST body in a loop instead of a single read() call,
  which PSR-7 allows to return short.

tests/Conformance/Fixtures/nginx.conf

@@ -3,6 +3,14 @@ server {
     server_name localhost;
     root /app;
 
+    # The two lifecycles are served side by side so one conformance run can
+    # target either without restarting the fixture: / speaks the handshake era
+    # (2025-11-25 and earlier), /stateless speaks the modern era (SEP-2575).
+    # Declared before `location /` so the prefix match wins.
+    location /stateless {
+        try_files $uri /tests/Conformance/server-stateless.php$is_args$args;
+    }
+
     location / {
         try_files $uri /tests/Conformance/server.php$is_args$args;
     }