Fix JSON diff viewer performance issue causing UI freeze with large payloads

- when the test report's payload data increases its freezing the ui.
- when there is match between expected and actual data it can load the 8k line which is max of(expected and actual data) but its taking time.
- when the expected and actual data is not matching so total length of both of 12k(6k expected + 6k actual) is causing ui freeze.

https://github.com/user-attachments/assets/d7dea59d-2aef-4463-9c0a-34938045bd08

Reproduce the freeze using the attached report and large expected and actual payloads, comparing matching data with the reported 8k-line case and mismatched data totaling about 12k lines. Trace the JSON diff viewer entry point and identify the rendering work that blocks the UI; done means these payload sizes load without freezing.

索引モデルが issue の本文から書いたものです。