Refactor SMS sender app to use async httpx and improved validation

[Manodeepray]

This PR refactors the SMS sender FastAPI app for improved performance, validation, and error handling. Key changes:

An overal summary

• Migrated from synchronous requests to asynchronous httpx.AsyncClient
• Added input validation using pydantic.Field
• Introduced async lifespan() for modern startup/shutdown handling
• Improved error handling with detailed logs and structured JSONResponse
• Updated requirements.txt

Changes i made

✅ Replaced requests.post with httpx.AsyncClient
✅ Updated endpoint to async def
✅ Added phone number pattern validation (+91... etc.)
✅ Introduced lifespan() for lifecycle management
✅ Added proper error responses with status codes
✅ Updated logging for Twilio and HTTP errors
✅ Created requirements.txt with minimal necessary deps

[pratik-mahalle]

Thank you for raising the pr, Our team will review the pr soon before that please make sure to sign the DCO.