Add analyzer to warn when enums are nested inside generic types

[Copilot]

This PR adds a new analyzer (diagnostic NEEG002) that detects when an enum decorated with [EnumExtensions] is nested inside a generic parent type and generates a warning, since this scenario is not currently supported.

Problem

Previously, the generator would attempt to generate extension methods for enums nested inside generic types, which would result in invalid code generation. For example:

public class GenericContainer<T>
{
    [EnumExtensions]
    public enum MyEnum  // This would generate invalid code
    {
        First,
        Second,
    }
}

Solution

The implementation adds:

1. New diagnostic descriptor EnumNestedInGenericType (NEEG002) with clear warning message
2. Hierarchy detection logic that walks up the containment chain to check if any parent type is generic
3. Enhanced generation pipeline that can handle both successful generation and diagnostic reporting
4. Comprehensive test coverage including deeply nested scenarios

Implementation Details

• Created EnumGenerationResult struct to handle both success and diagnostic cases
• Added GetEnumGenerationResult method that checks the type hierarchy before attempting generation
• When a generic parent is detected, reports the diagnostic and skips code generation
• Updated the source generation pipeline with ExecuteWithResult to handle diagnostics

Verification

The implementation correctly handles all scenarios:

• ✅ Enum directly nested in generic class → generates warning
• ✅ Enum in deeply nested hierarchy with generic ancestor → generates warning
• ✅ Enum in non-generic nested class → works normally
• ✅ Top-level enum → works normally

Fixes #115.

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