[kotlin] Fix jvm-ktor multipart formData for array of file uploads (#23297)

When a multipart/form-data endpoint has an array of binary file
parameters, the generated code calls `append(files)` with the
entire List<FormPart<InputProvider>> instead of iterating over
each element. This causes a compilation error:

  "Argument type mismatch: actual type is
   'List<FormPart<InputProvider>>', but 'FormPart<T>' was expected"

The root cause is that the `{{#isFile}}` block in the jvm-ktor
api.mustache template does not check for `{{#isArray}}`. When
`isFile` is true, the template unconditionally emits a single
`append(paramName)` call. The `{{#isArray}}` block that would
iterate is nested inside `{{^isFile}}`, so it is never reached
for file parameters.

PR #21056 (v7.13.0) correctly fixed the data type in the method
signature (`List<FormPart<InputProvider>>`), but did not update
the formData body to iterate over array file parameters.

This fix adds an `{{#isArray}}`/`{{^isArray}}` check inside the
`{{#isFile}}` block so that:
- Single file params: `paramName?.apply { append(paramName) }`
- Array file params: `for (x in paramName ?: listOf()) { append(x) }`

Fixes #18094

Author: kour0

modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-ktor/api.mustache

@@ -63,7 +63,14 @@ import com.fasterxml.jackson.databind.ObjectMapper
                         formData {
                     {{#formParams}}
                         {{#isFile}}
+                        {{#isArray}}
+                        for (x in {{{paramName}}} ?: listOf()) {
+                            append(x)
+                        }
+                        {{/isArray}}
+                        {{^isArray}}
                         {{{paramName}}}?.apply { append({{{paramName}}}) }
+                        {{/isArray}}
                         {{/isFile}}
                         {{^isFile}}
                         {{^isArray}}