Data-driven Testing

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

**Is your feature request related to a problem? Please describe.**
Testing an endpoint against many input combinations requires duplicating requests or hand-editing variables for each case.

**Describe the solution you'd like**
Run a collection or single request multiple times, iterating over rows from a CSV or JSON data file and substituting variables per iteration. Aggregate results (and assertions, #841) per row.

**Describe alternatives you've considered**
Manually duplicating requests; scripting a loop in the pre-request script.

**Additional context**
Critical for thorough API testing with realistic payloads. Builds on the Collection Runner (#774) and pairs with the Assertions tab (#841).

Start by reading the Collection Runner work in issue #774 and the Assertions tab work in issue #841 to understand their entry points and result handling. Define the scope for CSV and JSON row loading, per-iteration variable substitution, and aggregated results and assertions; the feature is done when a collection or single request can run through all data rows and report each row's outcome.

Written by the indexing model from the issue text.