Fix sensor forecast button to send train-start from available history

[Copilot]

• Identify forecast trigger payload and data range source
• Implement UI payload to send config.train-start from first available data
• Add/update tests for trigger payload construction regression
• Run targeted tests and repo validations
• Run CodeQL scan
• Open a separate PR linked to Forecast button trains on mostly empty 30-day window for sensors with little history #2186

[nhoening]

I did my test with 3 days data and confirm that this improves the situation a lot. Forecasts still look a bit different on the edges, but the base behavior is forecasted better.

Please add a changelog entry (standard procedure), then this can go into the upcoming release!

[BelhsanHmida]

Manual test passed.

I compared the sensor page Trigger forecast flow against the equivalent CLI command on 15-minute resolution test data.

The important thing I checked was that the button-triggered job no longer trains on the default 30-day window. The worker logs now show the training window starts from the available history instead:

Training cycle from 2023-01-01 00:15:00+00:00 to 2023-01-04 00:00:00+00:00 started ...
Prediction cycle from 2023-01-04 00:00:00+00:00 to 2023-01-06 00:00:00+00:00 started ...

I then ran the matching CLI forecast with the same train-start, start, and duration. The resulting forecasts are identical, which confirms the button now sends the intended config.train-start and behaves like the CLI path.

Also added a small follow-up fix so train-start is set for users who can trigger forecasts but do not have delete-data permission, plus test coverage for that rendered listener.