Subaru: handle transient API failures returning empty payloads

[ummon-v]

Subaru Connected Services API occasionally returns empty payloads on transient failures. Without validation, these responses surface as 0% SoC / 0 km range, causing evcc to act on bad data (e.g. starting unwanted charging sessions).

Implementation

• Wrap Subaru API status calls with caching (util.Cached) to avoid redundant requests and to retain last known good values
• Detect transient API failures where the response payload is empty (missing range unit) and signal api.ErrMustRetry instead of propagating invalid data
• Prevents 0% SoC from being reported during transient outages, which could incorrectly trigger minSoC charging

[sourcery-ai]

Hey - I've left some high level feedback:

• ValueInKilometers currently truncates the converted float to int64; consider using rounding (e.g. via math.Round) to avoid systematically under-reporting range after unit conversion.
• status() only checks for an empty Unit string to detect transient empty payloads; if the API can return other sentinel values (e.g. zero range with a non-empty unit) for failures, it may be worth broadening the validation so invalid data is consistently mapped to api.ErrMustRetry.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- ValueInKilometers currently truncates the converted float to int64; consider using rounding (e.g. via math.Round) to avoid systematically under-reporting range after unit conversion.
- status() only checks for an empty Unit string to detect transient empty payloads; if the API can return other sentinel values (e.g. zero range with a non-empty unit) for failures, it may be worth broadening the validation so invalid data is consistently mapped to api.ErrMustRetry.

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[andig]

this won't help as it does not clear the cache. The right thing to do would be not storing the invalid value in the first place.

[ummon-v]

I'm not 100% sure but it should be addressed with last push so zero can still be returned on a bad call, but it's never returned without the error, and the cache won't sit on it retrying immediately.

Do you think this is a good solution?

[andig]

remove. why wrap a wrapper?

[andig]

why change this? please don't make arbitrary unrelated changes!