refactor(browser): make sessionName optional, add selector filter, deduplicate session setup

- sessionName is now optional in open/exec; skips session restore when omitted
- add selector param to open: returns outerHTML or textContent of matched element
- extract session restore logic into private setupSession() to remove duplication
- replace per-exception instanceof checks with single HttpException base class check
- embed label into InternalServerErrorException message instead of logging separately
- update MCP tool schemas and HTTP DTOs to reflect optional sessionName and new selector
- add integration tests: sessionless open/exec, selector, selector+readerMode

test/__mocks__/jsdom.ts

@@ -1,6 +1,15 @@
+const mockElement = {
+  outerHTML: '<div id="mock">mock content</div>',
+  textContent: 'mock content',
+};
+
 export class JSDOM {
   constructor(public html: string, public options?: any) {}
   get window() {
-    return { document: {} };
+    return {
+      document: {
+        querySelector: (_selector: string) => mockElement,
+      },
+    };
   }
 }