refactor: replace fake_online_state integer with typed mcu2_online_ch…

[brianmay]

Summary

The fake_online_state field in Vehicle.Data used integer values 0–3 as magic numbers to track whether an apparent online event for MCU2-upgraded cars was a genuine wakeup or a brief subsystem check. The meaning of each value required reading multiple comments and mentally mapping integers to states.
This PR replaces it with a self-documenting atom enum field mcu2_online_check:

Old (fake_online_state)	New (mcu2_online_check)	Meaning
0	:idle	No MCU2 check in progress
1	:probing	Stream connected, awaiting first power reading
2	:confirmed_fake	Stream reported power=nil — subsystem wakeup only
3	:confirmed_real	Stream reported numeric power — genuine wakeup
The comment on the struct field has also been expanded to explain the MCU2 behaviour and the role of each value.
No behaviour changes. This is a pure rename/retype refactor — all pattern matches and state transitions are functionally identical to before.

Part of a larger simplification effort

This is step 1 of a planned series of mechanical refactors to make vehicle.ex easier to understand before merging the MCU2 logic into main. Subsequent steps will:

• Move drive/charge/update DB records out of state terms and into Data
• Eliminate the :start re-dispatch hub in favour of a direct private function
• Extract the driving offline sub-machine into its own named function

[brianmay]

This PR was entirely done with AI (opencode). So check it carefully.

So far, looks OK to me.

It also changed some of the log messages, which was a bit weird, but those changes good to me.

[brianmay]

I think failing Elixir tests is standard feature of this branch. Will work on fixing that after these expected 4 PRs.

This PR probably should be squashed and merged.

[micves]

I think failing Elixir tests is standard feature of this branch.

Indeed it is :) The whole concept of connecting the Stream and messing with the states during online has made quite some negative impact on the tests :(

I'm happy that some work is planned to get this closer to main.

Much better with the enum 👍
I'm a little against the name mcu2_online_check.
The check is done for all cars, but on all other non-upgraded-to-muc2 cars it will be confirmed_real straight away (as far as I have understood from people having e.g. Model 3 on the same teslamate instance as a Model S)

If the plan is to still keep it that way, could we go with a more generic name? Maybe pre_online_check ?

[brianmay]

Happy to change it to pre_online_check.

Will wait maybe a day and ensure everyone is happy with that before making the change.

[brianmay]

Never mind, I just made the change.

[USAFPride]

Let me know when you have it ready to test out. My time with a MCU2 car is coming to an end as a replacement has arrived.

[brianmay]

Now that is is merged into the mcu2-upgraded-cars branch, which should get built using.

ghcr.io/teslamate-org/teslamate:pr-4453

In theory nothing has changed with this PR (and subsequent PRs I am working on also like #5259 ), this is just refactoring of code, which should have the same exact behavior.