fix(security): harden gRPC stream WAF, MCP fs arg validation, and PAT server binding

- ServeRPC now applies real-IP + WAF interceptors to streaming RPCs
  (RequestTask, IOStream), not just unary calls; without them
  authHandler.check saw an empty real IP so brute-force BlockIP counters
  never keyed on a source and the WAF block table was bypassed at the
  stream entrypoint. Factored real-IP resolution into ctxWithRealIP shared
  by unary and stream paths.
- handleFsRead rejects negative offset/length; handleFsWrite validates
  if_match_sha256 is 64 hex chars before dispatch.
- createAPIToken always verifies each server_id exists (even for admins),
  so an admin cannot bind a PAT to a not-yet-created server id that a
  future server would auto-inherit.
- Add relay zero-length-chunk rejection regression test.

Co-authored-by: cloudcode <cloudcode@users.noreply.github.com>

cmd/dashboard/controller/api_token.go

@@ -120,14 +120,12 @@ func createAPIToken(c *gin.Context) (*model.APITokenCreateResponse, error) {
 			}
 			seenSrv[sid] = struct{}{}
 			deduped = append(deduped, sid)
-			if !callerIsAdmin(c) {
-				server, _ := singleton.ServerShared.Get(sid)
-				if server == nil {
-					return nil, errors.New("server not found")
-				}
-				if !server.HasPermission(c) {
-					return nil, errors.New("permission denied on server")
-				}
+			server, _ := singleton.ServerShared.Get(sid)
+			if server == nil {
+				return nil, errors.New("server not found")
+			}
+			if !callerIsAdmin(c) && !server.HasPermission(c) {
+				return nil, errors.New("permission denied on server")
 			}
 		}
 		req.ServerIDs = deduped