refactor(session): auto-create named browser sessions

- create and register named browser sessions on open/exec when missing

- align scenario step DTO/entity/service by removing sessionName usage

- update MCP scenario-step schemas to use optional title fields

server/src/mcp/mcp.service.ts

@@ -502,7 +502,7 @@ export class McpService {
             .min(0)
             .describe("Execution order (ascending)"),
           type: z.enum(["login", "exec", "sign"]).describe("Step type"),
-          sessionName: z.string().describe("Session name used by this step"),
+          title: z.string().optional().describe("Optional step title"),
           execCode: z
             .string()
             .optional()
@@ -569,7 +569,7 @@ export class McpService {
             .enum(["login", "exec", "sign"])
             .optional()
             .describe("New step type"),
-          sessionName: z.string().optional().describe("New session name"),
+          title: z.string().optional().describe("New step title"),
           execCode: z.string().optional().describe("New exec code"),
           validateCode: z.string().optional().describe("New validation code"),
         },