fix(mongo): migrate MongoDocumentsJsonView to QueryaButton and enable JSON syntax highlighting #1432

Summary

In lib/features/mongodb/mongo_documents_json_view.dart:

1. Line 95 uses legacy shadcn_flutter OutlineButton for the "Copy JSON" action in the toolbar instead of QueryaButton.secondary (part of chore(ui): remove the remaining shadcn buttons and add guard tests for the button standard #1339 button standardization).
2. The controller is instantiated as a plain material.TextEditingController(text: ...) rather than a QueryaHighlightController (via SyntaxHighlightService.createHighlightController(language: QueryaCodeLanguage.json)). Consequently, the Extended JSON viewer has no syntax highlighting and renders plain monochrome text.

Proposed Solution

1. Replace OutlineButton with QueryaButton.secondary.
2. Use SyntaxHighlightService.createHighlightController so syntax colors are rendered.

Acceptance Criteria

• "Copy JSON" toolbar action uses QueryaButton.secondary.
• Extended JSON documents display full JSON syntax highlighting.
• No direct OutlineButton or shadcn button imports in this file.