fix #4650 - adding validation for multichoice symbols

[JanKrivanek]

Problem

#4650
Choice symbols with multiple values allowed parse some strings (default values, choice value sent over API etc.) and split them on | or , chars - hence if those chars would be present in the actual choice values, results would be broken.

Solution

Adding template validation step, that validates choice symbols marked as allowing multiple values. Those choice symbols cannot have any choices that would contain the magic split chars

Checks:

• Added unit tests
• Added #nullable enable to all the modified files ?

[baronfel]

I like the wording here! It clearly describes the choice parameter that failed validation and why. One suggestion - I think we should include the valid separator character(s) in the message.

Suggested change

	<data name="Authoring_InvalidMultichoiceSymbol" xml:space="preserve">
	<value>Choice parameter {0} is invalid. It allows multiple values ('AllowMultipleValues=true'), while some of the configured choices contain separator characters. Invalid choices: {1}</value>
	<comment>{0} is the offending symbol name, {1} is a csv list of offending choice values</comment>
	</data>
	<data name="Authoring_InvalidMultichoiceSymbol" xml:space="preserve">
	<value>Choice parameter {0} is invalid. It allows multiple values ('AllowMultipleValues=true'), while some of the configured choices contain the separator character(s) {1}. Invalid choices: {2}</value>
	<comment>{0} is the offending symbol name, {1} is a set of separator characters, and {2} is a csv list of offending choice values</comment>
	</data>

[JanKrivanek]

Good improvement @baronfel! Incorporating.

[baronfel]

Minor feedback on the error message, but it looks very clear to me. Great work 👍