feat: capture mock request fields into test variables (v1 backport)

[keyclaim]

Backport of #325 to the v1 line.

What

Adds variablesToSet to mock definitions: values of the incoming request (JSON body, query, headers) are extracted with the template syntax of the template reply strategy ({{ .request.* }}) and become available to subsequent tests via {{ $varName }}.

mocks:
  backend:
    strategy: constant
    body: '{"ok": true}'
    variablesToSet:
      capturedUserId: '{{ .request.Json.user.id }}'
      capturedToken:  '{{ .request.Header "X-Auth" }}'
      capturedSource:
        template: '{{ .request.Query "source" }}'
        fromCall: first        # first | last (default)

Details

• works on nested definitions of composite strategies (uriVary, methodVary, sequence, basedOnRequest)
• fromCall: first|last selects the call to capture from when a definition is called multiple times (default: last)
• capture happens only for requests that passed requestConstraints; a missing JSON field is a test error
• on a name collision, the test response's variables_to_set takes precedence over a mock-captured value
• adds {{ .request.Header "name" }} to the mock template context

Backport note

Same change as #325, adapted to the pre-goccy/go-yaml loader on v1 (map[interface{}]interface{} mock definitions). Tests and docs identical.

See #325 for the full description.