fix: classify subscribe.* as core and reject x- extension types (#102 #106)

ExtensionNamespace::isCore() now treats subscribe./unsubscribe. prefixes as
core so subscribe.accepted/event/closed are no longer routed to extension
dispatch. isValidExtension() rejects any type whose first label starts with
x-, matching the documented reservation for transport-internal experimental
fields.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: nficano

src/Extensions/ExtensionNamespace.php

@@ -29,7 +29,7 @@ final class ExtensionNamespace
     private const array CORE_PREFIXES = [
         'session.', 'tool.', 'job.', 'stream.',
         'human.', 'permission.', 'lease.',
-        'subscribe', 'unsubscribe', 'artifact.',
+        'subscribe', 'subscribe.', 'unsubscribe', 'unsubscribe.', 'artifact.',
         'event.emit', 'log', 'metric', 'trace.span',
         'ping', 'pong', 'ack', 'nack',
         'cancel', 'interrupt', 'resume', 'backpressure',
@@ -61,6 +61,11 @@ public static function isValidExtension(string $type): bool
         if (self::isCore($type)) {
             return false;
         }
+        // The bare `x-` prefix is reserved for transport-internal
+        // experimental fields and MUST NOT be a long-lived envelope type.
+        if (str_starts_with($type, 'x-')) {
+            return false;
+        }
         // arcpx.<vendor>.<name>.v<n>  e.g. arcpx.example.v1
         // reverse-DNS prefix   e.g. com.acme.workflow.v2
         return preg_match('/^[a-z][a-z0-9-]*(?:\.[a-z0-9][a-z0-9-]*)+$/', $type) === 1;

tests/Unit/ExtensionsTest.php

@@ -23,6 +23,11 @@ public static function coreNames(): iterable
         yield 'ack' => ['ack'];
         yield 'metric' => ['metric'];
         yield 'log' => ['log'];
+        yield 'subscribe' => ['subscribe'];
+        yield 'subscribe.accepted' => ['subscribe.accepted'];
+        yield 'subscribe.event' => ['subscribe.event'];
+        yield 'subscribe.closed' => ['subscribe.closed'];
+        yield 'unsubscribe' => ['unsubscribe'];
     }
 
     #[DataProvider('coreNames')]
@@ -43,6 +48,8 @@ public static function extensionNames(): iterable
         yield 'uppercase first segment rejected' => ['ArcpX.example.v1', false];
         yield 'core prefix rejected' => ['session.foo', false];
         yield 'bare x- rejected (not a type name)' => ['x-foo', false];
+        yield 'dotted x- rejected (not a type name)' => ['x-foo.v1', false];
+        yield 'dotted x-experimental rejected' => ['x-experimental.foo.v1', false];
     }
 
     #[DataProvider('extensionNames')]