.Net Agents - Support role-override for ChatCompletionAgent

[crickman]

Motivation and Context

The "O*" series deep-reasoning models from OpenAI do not support System role. For certain versions, support was added for a Developer role.

Description

Allow ChatCompletionAgent make use of "O*" series models by supporting InstructionsRole that may be optionally overriden.

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 😄

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Comments suppressed due to low confidence (1)

dotnet/src/Agents/Core/ChatCompletionAgent.cs:59

• [nitpick] The new property 'InstructionsRole' defaults to AuthorRole.System; consider enhancing the XML documentation to clearly indicate that when targeting deep-reasoning models (which require 'developer'), users need to override this property accordingly.

public AuthorRole InstructionsRole { get; init; } = AuthorRole.System;

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