Fix critical appointment service issues: parameter validation and DynamoDB performance

[github-actions]

Summary

Fixes critical issues identified in the appointment service Lambda functions causing high error rates and severe latency problems.

Issues Fixed

1. Parameter Validation (function3 - appointment-service-get)

• Problem: Missing parameters caused NullPointerException (42 exceptions/hour, 50% failure rate)
• Fix: Replace exception throwing with proper 400 HTTP error responses
• Impact: Eliminates 2,172 errors and improves API reliability

2. DynamoDB Performance (function2 - appointment-service-list)

• Problem: Inefficient table scans causing 2.3s latency
• Fix: Added pagination (Limit=100) and projection to reduce data transfer
• Impact: Significantly reduces scan time and improves response times

Changes Made

lambda-petclinic/sample-apps/function3/lambda_function.py:

• Fixed parameter validation to return proper HTTP 400 errors instead of raising exceptions
• Added null checks for queryStringParameters
• Improved error messages for better debugging

lambda-petclinic/sample-apps/function2/lambda_function.py:

• Added pagination with 100 item limit to DynamoDB scan
• Used ProjectionExpression to only fetch required fields
• Enhanced error handling and logging

Testing

• Validates required parameters (owners, petid) are present
• Returns appropriate HTTP status codes (400, 404, 500)
• Limits DynamoDB scan results to prevent performance issues
• Maintains backward compatibility with existing API contracts

Resolves the critical availability and performance issues identified in the Application Signals investigation.