Summary

Fixes two bugs in the CloudFormation provisioner that broke Serverless Framework deployments.

Bug 1: Auto-named S3 bucket gets a new name on every stack update

When a CloudFormation stack is updated, AWS::S3::Bucket resources without an explicit BucketName (such as the ServerlessDeploymentBucket created by Serverless Framework) were incorrectly getting a new random bucket name on every update instead of preserving the existing one.

This caused Lambda functions referencing the bucket via {"Ref": "ServerlessDeploymentBucket"} to resolve to the new empty bucket instead of the one where the deployment zip was actually uploaded, resulting in Lambda functions created with no code.

Root cause: _RESOURCE_HANDLERS had no "update" entry for AWS::S3::Bucket. On stack update, _update_resource fell back to _provision_resource(type, physical_id, props, stack_name), passing the old physical resource ID where _s3_create expected a logical ID. _s3_create called _physical_name() which always generates a new random suffix, creating a brand new empty bucket.

Fix: Add _s3_update handler that preserves the existing physical resource ID (bucket name) when the bucket is updated in place (matching real AWS CloudFormation behavior). Also make _s3_create use setdefault to avoid wiping existing bucket data.

Bug 2: Lambda functions deployed via CFN missing CodeSize and proper CodeSha256

The CF provisioner _lambda_create never set CodeSize in the config dict and hardcoded CodeSha256 to "cfn-deployed". This caused UIs (like StackPort) to display "NaN undefined" for code size, even when the code was correctly loaded from S3.

Fix: Compute CodeSize and CodeSha256 from the actual zip bytes when available.

Reproduction

1. Deploy a Serverless Framework service to ministack using serverless-localstack
2. Deploy again (update)
3. Observe: Lambda functions have no code, "NaN undefined" code size, and a new empty S3 bucket is created on each update
4. S3 bucket list shows multiple deployment buckets, only the oldest has data

Test plan

• test_cfn_auto_named_s3_bucket_stable_across_updates — auto-named bucket keeps same name and objects across stack updates
• test_cfn_lambda_s3_ref_bucket_has_code_size — Lambda with Code.S3Bucket: {Ref: Bucket} reports correct CodeSize, real CodeSha256, and code is downloadable
• Existing CFN tests with explicit BucketName still pass (no regression)
• Full Serverless Framework deploy + redeploy validated end-to-end locally