Fixed TextMessage unmarshal JSON

[virgilio-a-cunha]

The TextMessageData.Variables field is currently defined as []TextMessageVariable, but it also needs to support JSON input where variables are provided in a map/object format, for example:

"variables": { "key": "value" } as documented

Currently, when the TextMessageReceived event includes variables in this object format, JSON unmarshalling fails, causing the event to be dropped.

This PR adds a custom UnmarshalJSON method for the TextMessageData struct to convert JSON object-style variables into the expected slice format. It also maintains backward compatibility with the existing JSON array structure.

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This change is