feat: add --express flag to sam delete

[koteshyelamati]

Closes #9115

Which issue(s) does this change fix?

#9115

Why is this change necessary?

#9106 added --express/--no-express support to sam deploy and sam sync for CloudFormation Express mode, but sam delete does not expose the same flag. For teams using express mode during development, delete is part of the same iteration loop: without express support on sam delete, teardown stays on the slower standard path even when the stack was created with --express.

How does it address the issue?

Mirrors the deploy/sync implementation for the delete path:

• sam delete gains --express/--no-express (default: off, backwards compatible). The flag is persisted to samconfig.toml via the existing --save-params machinery, consistent with deploy/sync.
• When enabled, DeleteContext passes DeploymentConfig={"Mode": "EXPRESS"} to every CloudFormation DeleteStack call (main stack, ECR companion stack, and the retain_resources retry paths).
• CfnUtils.delete_stack accepts an optional deployment_config and only includes DeploymentConfig in the boto call when provided, so non-express behaviour is byte-for-byte unchanged.
• After a successful express delete, a yellow notice is printed that some resources may still be removing in the background — consistent with the deploy/sync messaging from feat: add --express flag to sam deploy and sam sync #9106.
• The express option is registered in the delete help formatter (core/options.py).

The guided cleanup of S3 artifacts, ECR images, and companion stacks is unchanged — express mode only affects the CloudFormation stack deletion calls, which is what the current aws cloudformation delete-stack --deployment-config workaround bypasses.

What side effects does this change have?

None when the flag is not used: DeleteStack is called with exactly the same arguments as before.

Checklist

• Add input/output type hints to new functions/methods
• Write unit tests: flag plumbing from do_cli to DeleteContext, and CfnUtils.delete_stack with/without DeploymentConfig (all tests/unit/commands/delete and tests/unit/lib/delete pass)

[roger-zhangg]

Thanks for the contribution @koteshyelamati ! The change LGTM, I added integ test & schema update.