Python: Fix OpenAPI concept sample

[moonbox3]

Motivation and Context

An OpenAPI concept sample that we have is broken. This PR fixes it to run properly based on the current OpenAPI plugin handling.

Description

Fixes the broken OpenAPI sample.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[Copilot]

Pull Request Overview

This PR fixes the broken OpenAPI sample by updating the server, client, YAML configuration, and documentation to align with the current OpenAPI plugin handling.

• Updated the server docstring to clearly indicate its purpose.
• Modified the client to compute the absolute path for the OpenAPI specification and updated the kernel arguments.
• Changed the header parameter in the OpenAPI YAML from optional to required and updated the README instructions.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
python/samples/concepts/plugins/openapi/openapi_server.py	Updated docstring to "OpenAPI Sample Server" for clearer identification.
python/samples/concepts/plugins/openapi/openapi_client.py	Revised client code to compute the spec path and pass updated KernelArguments.
python/samples/concepts/plugins/openapi/openapi.yaml	Changed the header parameter to be required in accordance with sample requirements.
python/samples/concepts/plugins/openapi/README.md	Updated navigation instructions to reflect the current directory structure.

Comments suppressed due to low confidence (1)

python/samples/concepts/plugins/openapi/openapi_client.py:24

• [nitpick] The parameter name 'input' may be confused with Python's built-in function. Consider using a more descriptive name if it doesn't conflict with the intended API.

        input="hello world",