chore: Add an end to end device manager test

[allenporter]

Add an end to end test for the Device Manager. This exercises the case where there is no cache and the device looks up the device info from MQTT then connects locally over v1 protocol. We'll add additional scenarios in followup changes.

[Copilot]

Pull request overview

This PR adds an end-to-end test for the Device Manager that exercises the workflow where a device looks up device information from MQTT and then connects locally using the v1 protocol (without using cached data). The test uses mocked MQTT broker and local device connections to simulate real-world behavior.

• Adds comprehensive e2e test for Device Manager with MQTT and local connection flow
• Includes snapshot testing for request/response verification
• Adds v3 API endpoint mock to support newer API version

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
tests/e2e/test_device_manager.py	New end-to-end test file with Device Manager test, helper fixtures, and ResponseBuilder utility class
tests/e2e/snapshots/test_device_manager.ambr	Snapshot file capturing expected MQTT and local protocol message exchanges
tests/fixtures/web_api_fixtures.py	Added mock for v3/user/homes endpoint to support v3 API calls
tests/e2e/init.py	Registered web_api_fixtures plugin for e2e tests

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