[Server] Close the conformance gaps in the modern lifecycle (#447)

* [Server] Let the client gateway see modern-era capabilities

The six supports*() probes read session state only InitializeHandler wrote,
so every request served statelessly reported no client capabilities at all —
a tool guarding on them took the unsupported branch without a signal.
StatelessProtocol now writes the request's declaration under the same keys.

* [Server] Reject the RPCs 2026-07-28 removed

resources/subscribe and resources/unsubscribe were still dispatched and
answered 200 OK, recording subscriptions the modern era never reads. They
join initialize, ping and logging/setLevel in the era guard; the handlers
stay registered because the handshake era still serves them.

* [Server] Decode a wrapped Mcp-Name before comparing it

The Base64 sentinel was only unwrapped for Mcp-Param-*, so any resource URI
or tool name outside the header-safe ASCII set — which the spec explicitly
tells clients to wrap — was compared encoded and refused with -32020.

* [Server] Require the MCP-Protocol-Version header

It was only checked for contradicting the body, so omitting it entirely
passed — leaving the value intermediaries route on unenforced. Tied to the
header validator's presence, since that is what a header-bearing transport
installs and stdio carries its metadata inline.

* [Server] Read the modern-era request body whole

A single read() takes whatever the stream cares to give — 64 bytes from a
chunked transfer is legal — so an oversized-but-valid POST came back as a
parse error. Shares the legacy transport's incremental read as a trait.

* [Schema][Server] Answer a notification with a status, not a response

A body with no id was dispatched as a request and came back as a JSON-RPC
error carrying "id": "" — an id nobody issued, and on a path a client uses
to tell a modern server from a legacy one. Notifications now get 202 and no
body, and Error omits an id it could not read instead of emptying it.

* [Server] Stop emitting the error codes 2026-07-28 reserved

resources/read answered -32002, which this revision forbids; prompts/get
and completion/complete answered it for an unknown *name*, which it never
meant; tools/call answered -32601, which belongs to an unknown method.
Only the resources/read code is revision-gated — older peers expect -32002
there, and nowhere else. Also routes the -32021 capability exception out of
the prompt and resource handlers, which were swallowing it into -32603.

src/Server/Transport/StreamableHttpTransport.php

@@ -34,6 +34,8 @@ use Symfony\Component\Uid\Uuid;
  */
 class StreamableHttpTransport extends BaseTransport
 {
+    use ReadsBoundedBody;
+
     public const SESSION_HEADER = 'Mcp-Session-Id';
     public const PROTOCOL_VERSION_HEADER = 'Mcp-Protocol-Version';
 
@@ -293,33 +295,10 @@ class StreamableHttpTransport extends BaseTransport
 
     /**
      * Reads the request body, bounded by {@see self::$maxBodyBytes}.
-     *
-     * Returns the body contents, or `null` when the payload exceeds the cap. When
-     * the stream advertises a size we reject up-front; otherwise (e.g. chunked
-     * transfer with unknown size) we read incrementally and stop at the cap so an
-     * unbounded stream cannot exhaust memory.
      */
     private function readBody(StreamInterface $body): ?string
     {
-        $size = $body->getSize();
-        if (null !== $size && $size > $this->maxBodyBytes) {
-            return null;
-        }
-
-        $contents = '';
-        while (!$body->eof()) {
-            $chunk = $body->read(8192);
-            if ('' === $chunk) {
-                break;
-            }
-
-            $contents .= $chunk;
-            if (\strlen($contents) > $this->maxBodyBytes) {
-                return null;
-            }
-        }
-
-        return $contents;
+        return $this->readBoundedBody($body, $this->maxBodyBytes);
     }
 
     /**