[mqtt] Expose read-only trigger channels

[lsiepel]

Typed channels configured with trigger=true and without a commandTopic are now exposed as trigger channels. This allows framework consumers such as Main UI to distinguish them from state channels.

Typed trigger behavior remains backward compatible: incoming payloads are validated using the configured channel type before emitting a trigger event. Channels that also define a commandTopic retain state-channel metadata so linked Item commands can still be published to MQTT.

Adapted docs to:

• Recommend Type trigger for untyped MQTT trigger events.
• Explain the typed trigger=true behavior.
• Document the command-capable hybrid case.
• Clarify that Trigger String and Trigger Switch do not select an MQTT channel type and are unsupported.

Fixes: #19583

Disclaimer: tests and documentation created here with assistence from codex, supervised and reviewed by me, runtime code by me