Add response headers to mock

Currently, responses don't include the headers defined in the schema.

For example:
```yaml
paths:
/hello:
get:
summary: Hello world
responses:
'200':
description: OK
headers:
x-foo:
schema:
type: string
example: bar
content:
application/json:
schema:
type: object
properties:
foo:
type: string
```

Should return `x-foo: bar` as a response header.

No source file or test is named. Trace how the mock builds responses from the OpenAPI schema, starting with the response headers shown in the example, and verify that the x-foo header is returned with value bar for the documented /hello response.

Written by the indexing model from the issue text.