[BUG] array parameters in FormData should not be join to string by csv

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• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
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Description

api templates use wrong way to deal with array parameters in form body. for example:
line 165-177 at typescript-rxjs/apis.mustache:

        const formData = new FormData();
        {{#formParams}}
        {{#isArray}}
        if ({{> paramNamePartial}} !== undefined) {
            {{#isCollectionFormatMulti}}
            {{> paramNamePartial}}.forEach((element) => formData.append('{{baseName}}', element as any))
            {{/isCollectionFormatMulti}}
            {{^isCollectionFormatMulti}}
            formData.append('{{baseName}}', {{> paramNamePartial}}.join(COLLECTION_FORMATS['{{collectionFormat}}']));
            {{/isCollectionFormatMulti}}
        }

        {{/isArray}}

only if isCollectionFormatMulti is true, the array parameter will be add to FormData as an array, otherwise it will be join to a string.
according to DefaultCodegen::getCollectionFormat, isCollectionFormatMulti is true when style=form and explode=true, BUT "style" and "explode" are not valid for request body parameters.
ref: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.1.md#style-values
style and explode are for Parameter Object which define parameters in path, query, header or cookie, so you can never set style=form, explode=true for an array parameter in request body(FormData). manually add them will cause a "Schema validation: Property 'style' is not allowed" warning at ide.

openapi-generator version

checked 5.0.0, 6.0.0 and 7.5.0, I think the problem never been fixed at any version

OpenAPI declaration file content or url

Generation Details

Steps to reproduce

Related issues/PRs

Suggest a fix

remove isCollectionFormatMulti checking code for array parameters in FormData, like this:

        const formData = new FormData();
        {{#formParams}}
        {{#isArray}}
        if ({{> paramNamePartial}} !== undefined) {
            {{> paramNamePartial}}.forEach((element) => formData.append('{{baseName}}', element as any))
        }

        {{/isArray}}

Start in typescript-rxjs/apis.mustache around lines 165-177 and inspect how array form parameters are serialized into FormData. Generate a client from the multipart/form-data schema shown in the issue, then verify that each array element is appended separately rather than joined into one CSV string.