make http response var capture more generic

[theodore-s-beers]

allow capturing values from http test responses either using the json/jq system (pre-existing), or via regex against response body text

[Copilot]

Pull request overview

This PR expands HTTP test response variable capture so values can be extracted either from JSON using the existing jq-path mechanism, or from arbitrary response body text using a regex capture group.

Changes:

• Added BodyRegex to HTTP response variable definitions (bodyRegex in YAML).
• Updated HTTP variable parsing to support regex-based extraction and to validate variable configuration.
• Updated variable rendering and tests to reflect the new capture mode.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
render/variables.go	Uses a shared helper to describe response-variable sources (JSON path vs body regex) in rendered variable sections.
render/variables_test.go	Updates expected rendered output and adds coverage for regex-based response variables in the render layer.
client/lessons.go	Extends the response-variable schema with BodyRegex (yaml:"bodyRegex").
checks/http.go	Implements regex-based capture in parseVariables and tightens configuration validation; adjusts header-regex error handling.
checks/http_test.go	Adds tests for body-regex capture and for rejecting invalid/underspecified response-variable configs.

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[theodore-s-beers]

For the time being I want to err on the side of "don't flood the student with detail." We can always add detail back over time, if needed.

[hbeckmann]

Looks great! Also, thanks for adding tests 🙏