CAMEL-22737 Exception on unsupported content type

[xxxyyyz]

Description

Throw an AS2DecryptionException when an entity contains an unsupported content type.
EntityParser::parseEntityBody currently returns null when the entity content type is not explicitly handled by one of the switch cases. This leads to a NullPointerException when the caller does not check the response for null values (e.g. in method parseMultipartSignedEntityBody).
The pull request throws an exception of type AS2DecryptionException allows the error to propagate the calling stack, leading to a helpful response message.

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