Validate Response against OpenAPI Schema

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

**Is your feature request related to a problem? Please describe.**
After importing an OpenAPI spec (#621, #835), there is no way to check whether a live response actually conforms to the documented schema, so contract drift goes unnoticed.

**Describe the solution you'd like**
When a request originates from (or is linked to) an imported OpenAPI operation, validate the response body/status/headers against the spec's schema and surface mismatches inline (and as assertions in a run).

**Describe alternatives you've considered**
Hand-writing JSON-schema assertions in scripts; validating externally with a separate tool.

**Additional context**
Leverages the existing OpenAPI import. Pairs with the Assertions tab (#841) and Collection Runner (#774).

Start by tracing how imported OpenAPI operations are represented and connected to the Assertions tab and Collection Runner. Define the response body, status, and header checks, then verify that mismatches appear inline and become assertions during a run.

Written by the indexing model from the issue text.