cli http tests: allow response header capture

[theodore-s-beers]

need something like this for an upcoming course. there is a corresponding pr for the backend.

[Copilot]

Pull request overview

Adds support for capturing values from HTTP response headers into CLI variables (useful for flows like extracting a session id from Set-Cookie), and introduces an option to disable redirect following so learners can assert against intermediate redirect responses.

Changes:

• Extend HTTP lesson step schema to include ResponseHeaderVariables and FollowRedirects.
• Capture header-derived variables during runHTTPRequest and render them in the “Variables Saved/Missing” sections.
• Add tests covering response-header capture and redirect disabling.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
render/view.go	Refactors newline rendering in CLI output (switches to WriteByte / WriteString).
render/variables.go	Renders saved/missing variables sourced from response headers and adds header-variable descriptions.
render/variables_test.go	Updates variable-section tests to include response header variables.
client/lessons.go	Updates the lesson/step data model for ResponseHeaderVariables and FollowRedirects.
checks/http.go	Implements redirect disabling and response-header variable extraction during HTTP steps.
checks/http_test.go	Adds integration-style test for header capture + no-redirect behavior.

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