Extend Expression Builder for Projection expressions

[aanton-git]

Description

The AWS SDK for .NET does not currently provide a structured or fluent API for constructing ProjectionExpressions. Users must manually compose projection strings and manage ExpressionAttributeNames, which is error-prone and inconsistent with other AWS SDKs.

The Go SDK v2 includes a dedicated ProjectionBuilder as part of its DynamoDB expression package, enabling safe, composable construction of projection expressions with automatic attribute name aliasing.

Usage example

var projection = ProjectionExpressionBuilder
.New()
.WithName("parent.child[0].attribute1")
.AddName(NameBuilder.New("test.attribute2"))
.NamesList(NameBuilder.New("Attribute1"), NameBuilder.New("Attribute2"))
.Build();

Motivation and Context

This would:

• Reduce boilerplate and runtime errors
• Improve cross-SDK consistency
• Align .NET DynamoDB expression ergonomics with Go SDK patterns

Testing

Breaking Changes Assessment

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• [ x] 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
• [ x] 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

[Copilot]

Pull request overview

Adds a new fluent builder for DynamoDB ProjectionExpression construction in the DocumentModel expression-building framework, aiming to reduce manual string composition and make projection building consistent with existing expression builders.

Changes:

• Introduces ProjectionExpressionBuilder to build comma-separated projection expressions with automatic attribute name aliasing.
• Adds unit tests covering single-name, nested-path, special-character, and multi-name projection scenarios (plus empty-builder exception).
• Adds a DevConfig entry for DynamoDBv2 release notes/version bump metadata.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
sdk/src/Services/DynamoDBv2/Custom/DocumentModel/ExpressionBuilder.cs	Adds new ProjectionExpressionBuilder implementation integrated with existing ExpressionBuilder/aliasing mechanism.
sdk/test/Services/DynamoDBv2/UnitTests/Custom/ExpressionBuilderTests.cs	Adds unit tests validating projection expression statements and generated ExpressionAttributeNames.
generator/.DevConfigs/264e45e8-60ef-4e07-8f70-7fb4dc554186.json	Adds release metadata for the DynamoDBv2 change.

[GarrettBeatty]

can you update the pr description with example of how user can use it