Add tests for scene platform in Overkiz

[iMicknl]

Proposed change

Add tests covering the scene entity setup and activation using real fixtures.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:
• Link to developer documentation pull request:
• Link to frontend pull request:

Checklist

• I understand the code I am submitting and can explain how it works.
• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• I have followed the perfect PR recommendations
• The code has been formatted using Ruff (ruff format homeassistant tests)
• Tests have been added to verify that the new code works.
• Any generated code has been carefully reviewed for correctness and compliance with project standards.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• For the updated dependencies a diff between library versions and ideally a link to the changelog/release notes is added to the PR description.

To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[Copilot]

Pull request overview

Adds automated test coverage for the Overkiz scene platform, validating entity creation from real scenario fixtures and ensuring scene activation calls the expected client API.

Changes:

• Add snapshot-based tests to verify scene entities are created correctly from multiple scenario fixtures.
• Add service-call tests to verify scene.turn_on triggers execute_scenario with the correct scenario OID(s).
• Add Overkiz scenario JSON fixtures and corresponding Syrupy snapshots.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
tests/components/overkiz/test_scene.py	New tests covering scene entity setup (snapshots) and activation behavior (service calls).
tests/components/overkiz/snapshots/test_scene.ambr	New snapshots for entity registry entries and states created from scenario fixtures.
tests/components/overkiz/fixtures/scenarios/tahoma_switch.json	Adds scenario fixture data for TaHoma Switch to drive scene tests.
tests/components/overkiz/fixtures/scenarios/cozytouch.json	Adds scenario fixture data for Cozytouch to drive scene tests (multiple scenes).