Replace legacy params on SaveItem operations

[irina-herciu]

Description

This PR migrates the internal SaveAsync update flow to use DynamoDB’s modern expression-based API.

Specifically, it replaces usage of legacy UpdateItem request fields (AttributeUpdates, Expected, and ConditionalOperator) with:

• UpdateExpression
• ConditionExpression
• ExpressionAttributeNames
• ExpressionAttributeValues

This aligns the .NET SDK with current DynamoDB API recommendations and eliminates reliance on deprecated request patterns.

Motivation and Context

The current implementation of SaveAsync ultimately calls:

UpdateHelperAsync(UpdateItemDocumentOperationRequest, CancellationToken)

which still constructs UpdateItemRequest using legacy fields:

• AttributeUpdates
• Expected
• ConditionalOperator

These parameters are marked as legacy by AWS, with guidance to use expression-based APIs instead.

Migrating to expressions ensures:

• Alignment with modern DynamoDB best practices
• Consistency with other SDKs
• Improved flexibility for future enhancements

Testing

• Added/updated unit and integration tests covering:

  • Standard updates via SaveAsync
  • Conditional updates (previously using Expected)
  • Mixed update scenarios

Dry-run

• Dry-run ID:
• Status:
  • Pending
  • Completed successfully
  • Failed
• Failed bypass reason:

Breaking Changes Assessment

1. Identify all breaking changes including the following details:
   • What functionality was changed?
   • How will this impact customers?
   • Why does this need to be a breaking change and what are the most notable non-breaking alternatives?
   • Are best practices being followed?
   • How have you tested this breaking change?
2. Has a senior/+ engineer been assigned to review this PR?

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license