[Schema][Server] Preserve request id in JSON-RPC error responses instead of fabricating id:"" (#379)

* Preserve request id in JSON-RPC error responses instead of fabricating id:""

When the server received input it could not turn into a valid message, the
error response carried a fabricated empty-string id that was never in the
request, breaking JSON-RPC client correlation.

The id default for the error factories was '' and that default reached the
wire unchanged: unrecoverable parse errors emitted id:"", and for invalid-
but-parseable messages MessageFactory discarded the decoded id so it also
fell back to "".

Now unrecoverable parse errors (-32700) return id: null per JSON-RPC 2.0,
and invalid-but-parseable requests (-32600) preserve the original request
id. Error::$id / getId() and the $id parameter of forParseError() /
forInvalidRequest() are widened to string|int|null, and the recoverable id
is threaded through InvalidInputMessageException.

Fixes #333

* Preserve nullable request id consistently across Error factories and fromArray

- Widen the remaining for*() factories (forMethodNotFound, forInvalidParams,
  forInternalError, forServerError, forResourceNotFound) to string|int|null
  defaulting to null, so no error path can fabricate id:"" again. The only
  defaulting call site (ProtocolVersionMiddleware) now emits id:null instead
  of id:"" for a pre-parse rejection, which is the spec-correct value.
- Accept a null id in Error::fromArray(): require the id key to be present
  (JSON-RPC responses must carry it) but allow its value to be null, so a
  spec-compliant id:null error response round-trips and incoming error
  responses with id:null are accepted instead of rejected.
- Widen the @phpstan-type ErrorData id to string|int|null to match the ctor,
  getId(), and jsonSerialize() shape.

* Document intentional id recovery guard in MessageFactory

* Cover id: 0 and string ids in recoverable-id preservation test

* Fixed client handling of null-id error responses

tests/Unit/Client/ProtocolTest.php

@@ -112,6 +112,20 @@ final class ProtocolTest extends TestCase
         $this->assertCount(1, $logger->warnings);
     }
 
+    #[TestDox('stores an error response under its id so the pending request can be correlated')]
+    public function testErrorResponseWithIdIsStoredForItsPendingRequest(): void
+    {
+        $protocol = new Protocol();
+
+        $protocol->processMessage('{"jsonrpc": "2.0", "id": 7, "error": {"code": -32601, "message": "Method not found"}}');
+
+        $response = $protocol->getState()->consumeResponse(7);
+
+        $this->assertInstanceOf(Error::class, $response);
+        $this->assertSame(7, $response->getId());
+        $this->assertSame(Error::METHOD_NOT_FOUND, $response->code);
+    }
+
     private function createConfiguration(ProtocolVersion $protocolVersion): Configuration
     {
         return new Configuration(