Fix validation exception which occurs if the default aws region mismatches with the user's region_name

[jona62]

Fix validation exception which occurs if the default aws region mismatches with the user's region_name

Description of changes:
Fixed a validation exception that occurred when the default AWS region mismatched with the user's provided region_name. The issue was that region validation was happening after the boto3 session was created, which could cause conflicts.

Key fixes:

1. Moved region validation before session creation - This prevents validation errors from occurring after the session is already established
2. Made session parameter optional - Changed _validate_and_resolve_region to accept Optional[boto3.Session] instead of requiring a session
3. Simplified validation logic - Streamlined the region conflict detection to be more robust

Fixes this error

ValueError: Region mismatch: provided region_name 'us-west-2' does not match boto3_session region 'us-east-1'. Please ensure both parameters specify the same region or omit the region_name parameter to use the session's region.

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[codecov-commenter]

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Additional details and impacted files

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