[2.x] fix: Return JSON-RPC error for unknown server methods (#9002)

The fallback ServerHandler silently dropped requests with unknown
methods, violating the JSON-RPC spec. Clients like Metals that send
unrecognized requests would never receive a response, potentially
causing timeouts or hangs.

Return a -32601 (Method not found) error response instead.

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

main-command/src/main/scala/sbt/internal/server/ServerHandler.scala

@@ -30,7 +30,10 @@ object ServerHandler {
 
   lazy val fallback: ServerHandler = ServerHandler({ handler =>
     ServerIntent(
-      onRequest = { case x => handler.log.debug(s"Unhandled request received: ${x.method}: $x") },
+      onRequest = { case x =>
+        handler.log.debug(s"Unhandled request received: ${x.method}: $x")
+        handler.jsonRpcRespondError(Some(x.id), -32601, s"Unknown method: ${x.method}")
+      },
       onResponse = { case x => handler.log.debug(s"Unhandled response received") },
       onNotification = { case x =>
         handler.log.debug(s"Unhandled notification received: ${x.method}: $x")

server-test/src/test/scala/testpkg/ResponseTest.scala

@@ -79,6 +79,16 @@ class ResponseTest extends AbstractServerTest {
     }
   }
 
+  test("unknown method returns error") {
+    val id = svr.session.nextId()
+    svr.session.sendJsonRpc(id, "build/foo", "{}").get
+    val response = svr.session.waitForResponseMsg(10.seconds, id).get
+    assert(
+      response.error.exists(_.code == -32601),
+      s"Expected method-not-found error, got: $response"
+    )
+  }
+
   private def neverReceiveResponse(
       duration: FiniteDuration
   )(predicate: sbt.internal.protocol.JsonRpcResponseMessage => Boolean): Unit =