Added Disease Diagnosis API in C# - #3

Description
This pull request adds a new sample project named DiseaseDiagnosisAPI to the samples-csharp repository as part of the Keploy API Fellowship - Task 2. The API is built with ASP.NET Core Web API and allows users to POST symptom details and receive a possible diagnosis based on simple rule-based logic.

##What’s Included

• DiseaseDiagnosisAPI/: New folder containing:
  • Controller for POST endpoint (/DiseaseDiagnosis)
  • Basic diagnosis logic based on input symptom text
  • Sample response formats
  • Minimal model class for request body
• README.md: Project description, usage guide, sample inputs

Tech Stack

• ASP.NET Core 6
• C#
• JSON-based API

Example Request

POST /DiseaseDiagnosis

{
 "symptoms": "fever and cough"
}