Extract BDA region from profile ARN instead of separate env var

[laurencegoolsby]

Context

The app requires a BDA_REGION env var to be configured separately in the infra. This is redundant since the region is already embedded in the BDA_PROFILE_ARN (e.g., arn:aws:bedrock:us-east-1:123456789012:data-automation-profile/us.data-automation-v1).

Relates to navapbc/platform-test#278

Implementation

• Parse the region from BDA_PROFILE_ARN at runtime (split on :, extract index 3)
• Remove BDA_REGION env var from app code and infra config
• Update tests to remove BDA_REGION fixtures