test: add backwards compatibility test for step handlers without Context parameter

[Copilot]

Summary

Adds test coverage for the backwards compatibility logic that allows step handlers to omit the Context parameter.

Motivation

The backwards compatibility implementation in Invoke-IdlePlanObject.ps1 (lines 376-379) conditionally passes the Context parameter only when handlers support it, but no test validated this with a handler that actually omits Context.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Refactoring / internal improvement

Changes

• Added Invoke-IdleTestLegacyStep test handler that only accepts $Step (no $Context)
• Added test "supports step handlers without Context parameter (backwards compatibility)" that validates execution with legacy handler signature
• Added cleanup for new test handler in AfterAll block

Testing

• Unit tests
• Contract tests
• Manual testing

How to test & review

Run Invoke-Pester -Path ./tests/Invoke-IdlePlan.Tests.ps1 and verify the new test passes. The test validates that:

1. Legacy handlers without Context parameter can be invoked
2. The engine completes successfully without attempting to pass Context
3. Step result is returned with expected status

Checklist

• Code follows STYLEGUIDE.md
• Tests added or updated
• Documentation updated
• No UI/auth logic added to IdLE.Core
• No breaking changes without discussion

Related Issues

Addresses feedback on PR #81: #81 (comment)

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