fix(cloudwatch-mcp-server): respect AWS_REGION before defaulting to us-east-1⁷

[AiGOVERNOR]

Fixes #1692

Summary

This PR fixes region detection in the CloudWatch MCP Server.
It ensures that the service honors AWS_REGION and AWS_DEFAULT_REGION environment variables before defaulting to us-east-1.

Changes

• Added get_effective_region() helper for consistent region resolution.
• Updated CloudWatch Logs client creation to use detected region.
• Region precedence: argument → environment → boto3 session → fallback us-east-1.

User experience

Before: The server always defaulted to us-east-1, ignoring environment settings.
After: The server correctly uses the configured AWS region, improving portability for multi-region deployments.

Checklist

• I have reviewed the contributing guidelines
• I have performed a self-review of this change
• Changes have been tested locally with and without AWS_REGION set
• Changes are documented in code comments

Is this a breaking change? (Y/N)
N

RFC issue number: N/A

Acknowledgment

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution under the terms of the project license.