Add backward-compatible JSON field parsing to asset and sensor methods

[Copilot]

The FlexMeasures API returns JSON-encoded fields (attributes, flex_context, flex_model) as strings. Methods like get_assets(), get_asset(), get_sensor(), and get_sensors() previously returned these as-is, requiring manual parsing by callers.

Changes

New parameter parse_json_fields (default: None)

• None (default): Returns JSON strings with deprecation warning
• True: Parses JSON strings into Python dicts
• False: Returns JSON strings without warning

Example usage:

# Old behavior (with deprecation warning)
assets = await client.get_assets()
attrs = json.loads(assets[0]["attributes"])  # Manual parsing required

# New behavior (opt-in)
assets = await client.get_assets(parse_json_fields=True)
attrs = assets[0]["attributes"]  # Already a dict

# Explicit old behavior (no warning)
assets = await client.get_assets(parse_json_fields=False)

Implementation details:

• Added helper functions _parse_json_field(), _parse_asset_json_fields(), _parse_sensor_json_fields()
• Silently handles malformed JSON by leaving fields as strings and logging at debug level
• Updated internal get_asset() call in scheduler code to use parsed fields
• Added test coverage for both parsing modes

Affected fields:

• Assets: attributes, flex_context, flex_model
• Sensors: attributes

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