Support new forecasting endpoints (/sensors/{id}/forecasts/trigger and /sensors/{id}/forecasts/{uuid})

[Copilot]

FlexMeasures server v0.31.0 introduced POST /sensors/{id}/forecasts/trigger and GET /sensors/{id}/forecasts/{uuid} for async forecast job execution. This adds client-side support for both.

New methods

• trigger_forecast(sensor_id, ...) — POSTs to /sensors/{id}/forecasts/trigger, returns forecast job UUID. Training params (train_start, train_period, retrain_frequency, regressors, etc.) are serialized into a nested config dict per the server schema.
• get_forecast(sensor_id, forecast_id) — GETs /sensors/{id}/forecasts/{uuid}, polling on 202 until the job completes (200). Returns {values, start, duration, unit}.
• trigger_and_get_forecast(sensor_id, ...) — Convenience wrapper combining both.

Usage

# Trigger and poll until complete
forecast = await client.trigger_and_get_forecast(
    sensor_id=5,
    start="2025-01-05T00:00:00+00:00",
    end="2025-01-07T00:00:00+00:00",
    train_start="2025-01-01T00:00:00+00:00",
    retrain_frequency="PT24H",
    future_regressors=[2, 3],
)
# {"values": [...], "start": "...", "duration": "PT48H", "unit": "kW"}

# Or step-by-step
forecast_id = await client.trigger_forecast(sensor_id=5, start="...", end="...")
forecast = await client.get_forecast(sensor_id=5, forecast_id=forecast_id)

Requires FlexMeasures server ≥ 0.31.0. get_forecast respects the client's existing polling_timeout, polling_interval, and max_polling_steps settings.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Support new forecasting endpoints</issue_title>
<issue_description>New forecasting API endpoints were introduced in the FlexMeasures server: [POST] /sensors/(id)/forecasts/trigger <api/v3_0.html#post--api-v3_0-sensors-id-forecasts-trigger>_ and [GET] /sensors/(id)/forecasts/(uuid) <api/v3_0.html#get--api-v3_0-sensors-id-forecasts-uuid>_ to forecast sensor data [see PR FlexMeasures/flexmeasures-client#1813 <https://www.github.com/FlexMeasures/flexmeasures/pull/1813>, PR FlexMeasures/flexmeasures-client#1823 <https://www.github.com/FlexMeasures/flexmeasures/pull/1823> and PR FlexMeasures/flexmeasures-client#1917 <https://www.github.com/FlexMeasures/flexmeasures/pull/1917>_]. We need to support these endpoints in the FlexMeasures client.</issue_description>

<agent_instructions>Make sure to use #1917 for the latest schema versions! And call the test-specialist as a sub-agent in your task. Make sure the test-specialist actually creates new tests and, super important, runs them successfully (not just says it ran them successfully by inspecting code).</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Support new forecasting endpoints #179

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