Add incoming external bus messages for matter

[TimoPtr]

Summary

Add incoming external bus messages for matter and mapping into the ViewModel with dumb logs to prepare the impl of the matter onboarding flow.

Checklist

• New or updated tests have been added to cover the changes following the testing guidelines.
• The code follows the project's code style and best_practices.
• The changes have been thoroughly tested, and edge cases have been considered.
• Changes are backward compatible whenever feasible. Any breaking changes are documented in the changelog for users and/or in the code for developers depending on the relevance.

[Copilot]

Pull request overview

This PR introduces new incoming external bus message types for Matter commissioning and Thread credential import, wiring them through JSON parsing into FrontendMessageHandler events and (currently) a placeholder branch in FrontendViewModel.

Changes:

• Added matter/commission and thread/import_credentials incoming message models with serialization support.
• Extended FrontendMessageHandler to translate these messages into new FrontendHandlerEvent variants.
• Added unit tests covering message parsing and handler event emission for both message types (with/without id).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
app/src/main/kotlin/io/homeassistant/companion/android/frontend/externalbus/incoming/IncomingExternalBusMessage.kt	Adds new incoming message types for Matter/Thread external bus commands
app/src/main/kotlin/io/homeassistant/companion/android/frontend/handler/FrontendMessageHandler.kt	Maps new incoming messages to handler events (and logs receipt)
app/src/main/kotlin/io/homeassistant/companion/android/frontend/handler/FrontendHandlerEvent.kt	Introduces new handler events representing Matter commissioning + Thread credential import requests
app/src/main/kotlin/io/homeassistant/companion/android/frontend/FrontendViewModel.kt	Adds placeholder handling that currently logs and does not complete the request
app/src/test/kotlin/io/homeassistant/companion/android/frontend/externalbus/incoming/IncomingExternalBusMessageTest.kt	Adds JSON deserialization tests for the new message types
app/src/test/kotlin/io/homeassistant/companion/android/frontend/handler/FrontendMessageHandlerTest.kt	Adds handler event emission tests for the new message types