Default to OAS-compliant handling of additionalProperties

[ctreatma]

This changes the default value for disallowAdditionalPropertiesIfNotPresent from true to false, which means that the generator will now, by default, produce code that complies OpenAPI standards with respect to the handling of additionalProperties.

Note that a number of example configs did not have to be updated as a result of this change, which indicates that the disallowAdditionalPropertiesIfNotPresent option had no effect on the generators used in those examples.

Closes #21169

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[ctreatma]

@wing328 I took a crack at swapping the default value of disallowAdditionalPropertiesIfNotPresent so that, in a future major version, openapi-generator can default to generating OAS-compliant code instead of defaulting to the old non-compliant behavior. I'm running into some unexpected changes in the sample code and could use your help understanding what's happening there.

[ctreatma]

@wing328 In the bin/configs/python-fastapi.yaml, as in many other sample configs, I added disallowAdditionalPropertiesIfNotPresent: true to avoid a PR that has massive changes across all examples. I expected that the samples would have 0 changes as a result, because true was the default value prior to this PR.

[ctreatma]

This is the sample code corresponding to the config I mentioned in #21170 (comment).

This code is being updated to throw errors when there are additional properties, but since the old default value was disallowAdditionalPropertiesIfNotPresent=true, these models should have already been throwing errors before I made my change. I'm not sure what I'm missing here. Is there some hidden third case for some languages when disallowAdditionalPropertiesIfNotPresent isn't explicitly set to a true or false?

[ctreatma]

This change in the README for one of the C# samples seems to reinforce the idea that the default value for disallowAdditionalPropertiesIfNotPresent is a hidden third option rather than true.

[ctreatma]

I'm curious: why also set the default value here? Does the default value not flow through automatically via the CliOption?