fix(lang): include mobile and skip refs in step dependency graph

[euskadi31]

StepDependencies only walked When, Request, Expect, Capture and Keyword expressions to extract implicit result.<step> references for the DAG. It never traversed step.Mobile nor step.SkipRules, so a mobile step referencing a previous keyword/HTTP step (e.g.
input_text { value = result.user.email }) had no recorded dependency on user. The DAG placed both steps in the same topological layer and ran them in parallel; the mobile evaluator then observed result.user as the empty object pre-populated by NewScenarioState and failed with the misleading HCL error "Unsupported attribute" in ~100us, before the keyword had a chance to write its outputs. The same latent bug applied to skip rules referencing other steps' results.

Refactor StepDependencies into focused helpers (collectRequestRefs, collectExpectRefs, collectMobileRefs, collectMobileExpectRefs, collectSkipRefs) to keep gocyclo under the v2 lint threshold while covering every Expression in MobileStep (Platform, Target, Launch.ClearState, each Action's five fields, every Expect.* slice with their ID/Expected/Timeout/Interval fields) and every Expression in SkipRule (Condition, Reason, OS, Arch, EnvSet, Env).

Adds four unit tests in refs_test.go for the new walks and a DAG-level integration test in runner_test.go that builds a keyword step followed by a mobile step referencing it and asserts buildLayers produces two distinct layers in the expected order.

[Copilot]

Pull request overview

This PR fixes step dependency graph construction by ensuring StepDependencies also traverses expressions inside step.Mobile and step.SkipRules, so result.<step> references in those blocks correctly enforce execution ordering (preventing unintended parallel execution and misleading early HCL evaluation failures).

Changes:

• Refactors StepDependencies into helper walkers for request/expect/mobile/skip rule expressions and adds the missing traversals.
• Adds unit tests covering mobile action refs, mobile expect refs, mobile platform/launch refs, and skip rule refs.
• Adds an integration-style test asserting buildLayers orders a mobile step after a keyword step it references via result.user.*.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
internal/lang/refs.go	Adds request/expect/mobile/skip expression walkers so result.<step> refs are included in step DAG dependencies.
internal/lang/refs_test.go	Adds unit tests ensuring mobile and skip-rule expressions contribute implicit step dependencies.
internal/runtime/runner_test.go	Adds a DAG layering test proving mobile steps referencing prior steps are scheduled in later layers.

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