fix(step): Use async context for stepper context

[rkaraivanov]

Description

Use async context for stepper context

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Checklist

• My code follows the project's coding standards
• I have tested my changes locally

[Copilot]

Pull request overview

Replaces the synchronous @consume decorator on IgcStepComponent with the project's createAsyncContext controller so that the stepper context is correctly bound when a step is connected before its parent stepper exists, then later re-parented.

Changes:

• Switch step.ts from @lit/context's @consume decorator to the createAsyncContext helper that defers ContextConsumer creation until after updateComplete.
• Add a regression test verifying that a step connected to the document before the stepper still binds context correctly after being adopted by a stepper.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/components/stepper/step.ts	Removes @consume decorator and uses createAsyncContext controller, assigning the resolved context via callback.
src/components/stepper/stepper.spec.ts	Adds a new "Context binding" test to verify the deferred context binding after step re-parenting.