[PHP][php-nextgen] Fix usage of enums in parameters (Fix #20086)

[JulianVennen]

Using enums in path, header or query parameters with the php-nextgen generator currently causes type errors like the following:

PHP Fatal error:  Uncaught TypeError: OpenAPI\Client\ObjectSerializer::toPathValue(): Argument #1 ($value) must be of type string, OpenAPI\Client\Model\IntEnum given, called in /tmp/openapi-tests/src/Api/DefaultApi.php on line 1183 and defined in /tmp/openapi-tests/src/ObjectSerializer.php:165
Stack trace:
#0 /tmp/openapi-tests/src/Api/DefaultApi.php(1183): OpenAPI\Client\ObjectSerializer::toPathValue()
#1 /tmp/openapi-tests/src/Api/DefaultApi.php(1051): OpenAPI\Client\Api\DefaultApi->pathIntPGetRequest()
#2 /tmp/openapi-tests/src/Api/DefaultApi.php(1033): OpenAPI\Client\Api\DefaultApi->pathIntPGetWithHttpInfo()
#3 /tmp/openapi-tests/dev.php(11): OpenAPI\Client\Api\DefaultApi->pathIntPGet()
#4 {main}
  thrown in /tmp/openapi-tests/src/ObjectSerializer.php on line 165

Example spec: https://gist.github.com/JulianVennen/237b96487d45bbe24b3b174e0e7a758c

This PR fixes those errors by passing the value to these functions instead.

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[wing328]

what about inline enum? shall we use isEnumOrRef instead?

[JulianVennen]

An inline enum for that would look like this, right?

paths:
  /path-int/{p}:
    get:
      parameters:
        - name: p
          in: path
          required: true
          description: Integer option in path
          schema:
            type: integer
            enum:
              - 0
              - 1
              - 2
            x-enum-varnames:
              - ZERO
              - ONE
              - TWO
      responses:
        '200':
          description: OK

That does not generate any model classes or constants in the php-nextgen generator.
The type of the function parameter is just int or string so adding ->value would be incorrect.

[wing328]

👌

let's give it a try