`idl:` CDR compatibility for unbounded `sequences` & `list`

[Splinter1984]

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According to issue #2247, an unbounded sequence and a list of the same subtype have incompatible CDR(v2) as a result of the serialisation operation. By following thought from https://github.com/eclipse-cyclonedds/cyclonedds/issues/2247#issuecomment-3068601131, this commit changes serialisation steps to follow existing sequence ser. process.