Critical Security Vulnerabilities in SDK Credential Management

## Overview
This issue documents critical security vulnerabilities identified in the Bedrock AgentCore SDK related to credential management, endpoint validation, and token handling.

## Vulnerabilities Identified
- **Endpoint Validation**: Hardcoded endpoints without proper validation allow potential injection attacks
- **Token Management**: Lack of proper token lifecycle management leading to potential token leaks
- **Input Validation**: Missing validation for user inputs creating security risks
- **Credential Exposure**: Sensitive credentials potentially exposed in log files

## Impact
These vulnerabilities could lead to:
- Unauthorized access to AWS resources
- Credential leakage
- Potential injection attacks
- Non-compliance with AWS security best practices

## Resolution
A comprehensive security framework has been implemented in PR #16 that addresses all identified vulnerabilities with:

- AWS domain pattern validation
- Thread-safe token lifecycle management
- Input sanitization and validation
- Secure logging practices

## Related PR
This issue is being addressed in PR #16: Security & Credential Management Enhancements

Start with PR #16, which the issue identifies as implementing the resolution, and review its changes against the four listed vulnerabilities. Confirm whether endpoint validation, token lifecycle, input validation, and secure logging are covered; the issue provides no files or tests for independent work.

Written by the indexing model from the issue text.