Fix: Corrected Validation Logic and Exception Handling for validate.py and SamTemplateValidator

[Pranaykarvi]

Description:

This pull request addresses and fixes a validation bug in the SamTemplateValidator and the associated CLI command used to validate AWS SAM templates. The following changes have been made:

• Updated the SamTemplateValidator to handle local CodeUri and ImageUri properly by replacing them with fake S3 URIs for SAM translation.
• Fixed an issue in the validate.py file where the region or credentials weren’t being handled correctly during SAM template validation.
• Modified the cli.py command to ensure the validation process is smooth and error-free when interacting with AWS services.

Changes:

• SamTemplateValidator: Replaced local CodeUri and ImageUri with S3 URIs for correct SAM translation.
• validate.py: Fixed region and credentials handling issues.
• cli.py: Updated the command to handle validation better and give clear error messages.

How to test:

1. Run the sam validate command with a sample SAM template to ensure that it correctly validates and processes local CodeUri and ImageUri.
2. Test with both valid and invalid templates to check if proper error messages are displayed.
3. Run the lint command to ensure that SAM template linting works as expected.

Related Issues:

• Bugfix for invalid region and credential handling during SAM template validation.

Notes:

• Ensure that you have your AWS credentials configured before testing the validation.
• If you encounter any issues, refer to the error messages to resolve credential or region-related problems.

fixes #7973

[valerena]

Hi. Thanks for the contribution! It looks like you missed to commit the changes from cli.py, alongside other of the changes you describe. The only changes visible right now are just comments/format, and handling of None returned by the transform. Please check the changes and make sure that you commited everything you meant to.