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

Issue が解決策の実装として特定している PR #16 から始め、4 つの列挙された脆弱性に照らしてその変更を確認してください。エンドポイントの検証、トークンのライフサイクル、入力の検証、安全なロギングがカバーされているか確認してください。issue には独立した作業のためのファイルやテストはありません。

索引モデルが issue の本文から書いたものです。