.Net Agents - Align with expectations when arguments override specified.

[crickman]

Motivation and Context

When default KernelArguments are specified for KernelAgent.Arguments, it is likely expected that when override arguments parameter is provided that this override doesn't necessarily invalidate the default KernelArguments.ExecutionSettings.

Description

Provide performant merge logic for default and override KernelArguments for all invocations.
This merge preserves any default values not specifically included in the override...including both ExecutionSettings and parameters.

Solves this (common) pattern:

ChatCompletionAgent agent =
    new()
    {
        Name = "SampleAssistantAgent",
        Instructions =
            """
            Something something.
            
            Something dynamic: {{$topic}}

            The current date and time is: {{$now}}. 
            """,
        Kernel = kernel,
        Arguments =
            new KernelArguments(new AzureOpenAIPromptExecutionSettings() { FunctionChoiceBehavior = FunctionChoiceBehavior.Auto() })
            {
                { "topic", "default topic" }
            }
    };
    
KernelArguments arguments =
    new()
    {
        { "now", $"{now.ToShortDateString()} {now.ToShortTimeString()}" }
    };
await foreach (ChatMessageContent response in agent.InvokeAsync(history, arguments))
{
    ...
} 	

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 😄