Add context clarification comments to interface method code snippets

[Copilot]

The code snippets in the default interface methods tutorial were showing interface method implementations without proper context, making it unclear to readers that these methods belong within interface declarations.

Problem:
When documentation readers viewed the code examples, they saw method implementations like:

public decimal ComputeLoyaltyDiscount()
{
    DateTime TwoYearsAgo = DateTime.Now.AddYears(-2);
    if ((DateJoined < TwoYearsAgo) && (PreviousOrders.Count() > 10))
    {
        return 0.10m;
    }
    return 0;
}

Without any indication that this code should be declared within the ICustomer interface.

Solution:
Added clarifying comments to all relevant code snippets that specify:

• The interface or class where each method belongs
• The filename for reference

Now readers see:

// This method belongs in the ICustomer interface (ICustomer.cs)
public decimal ComputeLoyaltyDiscount()
{
    DateTime TwoYearsAgo = DateTime.Now.AddYears(-2);
    if ((DateJoined < TwoYearsAgo) && (PreviousOrders.Count() > 10))
    {
        return 0.10m;
    }
    return 0;
}

Files changed:

• ICustomer.cs: Added context comments to SnippetLoyaltyDiscountVersionOne, SnippetLoyaltyDiscountVersionTwo, and SnippetFinalVersion
• SampleCustomer.cs: Added context comment to SnippetOverrideAndExtend

The changes are minimal and non-breaking - only clarifying comments were added without modifying any functional code.

Fixes #29643.

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[BillWagner]

This LGTM and is ready for final review.