Python: Respect FunctionChoiceBehavior filters in OpenAI responses agent tools

[ymuichiro]

issue: #13234

Summary

• honor FunctionChoiceBehavior filters when retrieving kernel function metadata

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 ensures that FunctionChoiceBehavior filters are properly respected when retrieving kernel function metadata in OpenAI responses agent tools. Previously, the code was ignoring these filters and always returning all available functions.

• Replaced the TODO comment with proper implementation of FunctionChoiceBehavior filter handling
• Added early returns for cases where kernel functions are disabled or no plugins exist
• Used conditional logic to apply filters when retrieving function metadata

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