Assertions Tab (No-Code Tests)

> ℹ️ _This issue was partially AI-generated and may contain technical inaccuracies._

**Is your feature request related to a problem? Please describe.**
Response testing currently requires writing JavaScript in the scripting editor (#710). Non-developers and quick checks are blocked by that barrier.

**Describe the solution you'd like**
Add an Assertions tab with a no-code UI to define checks such as:
- status code equals / is in range
- response time below a threshold
- JSON path value equals / contains / exists
- header present / equals
Assertions run automatically after each response and show pass/fail inline.

**Describe alternatives you've considered**
Scripting-only assertions (current state); a snippet library that generates test scripts from a form.

**Additional context**
Scripting stays available for advanced cases. Should integrate with the eventual Collection Runner (#774) so assertion results aggregate across a run.

Start by reading the current scripting-based response testing described in #710 and the Collection Runner context in #774. Determine how response results and editor tabs are currently handled, then clarify the supported assertion types and aggregation behavior. Done means a no-code Assertions tab runs the defined checks after each response and shows pass/fail results inline.

Written by the indexing model from the issue text.