[BUG][java-helidon-server] PartsUtils#partsMap does not work correctly

Description

When using a multipart form data upload the generated service uses PartsUtils#partsMap to grab all the parts by name then validates and passes the parts to the generated method.

However, when iterating over MultiPart Helidon will finish() the previous part before returning the next one, causing an error when accessing the parts from the Map

Cannot request input stream more than once

openapi-generator version

7.15.0

OpenAPI declaration file content or url

paths:
  "/mutipart":
    post:
      requestBody:
        required: true
        content:
          "multipart/form-data":
            schema:
              type: object
              properties:
                csvFile:
                  type: string
                  format: binary
              required:
                - csvFile
            encoding:
              csvFile:
                contentType: text/csv

Generation Details

Using Maven plugin

      <plugin>
        <groupId>org.openapitools</groupId>
        <artifactId>openapi-generator-maven-plugin</artifactId>
        <executions>
          <execution>
            <goals>
              <goal>generate</goal>
            </goals>
            <configuration>
              <generatorName>java-helidon-server</generatorName>
              <generateModels>false</generateModels>
              <configOptions>
                <fullProject>false</fullProject>
                <library>se</library>
                <useAbstractClass>true</useAbstractClass>
              </configOptions>
            </configuration>
          </execution>
        </executions>
      </plugin>

Steps to reproduce

Try to read the data from a ReadablePart in a generated service

Start with PartsUtils#partsMap and reproduce the issue using the provided multipart OpenAPI declaration and Maven plugin configuration. Trace how the generated service accesses ReadablePart values after iteration over MultiPart. Done means generated multipart services can read parts from the map without the “Cannot request input stream more than once” error.