Add BindingBase Bind overload + tests for nested path propagation

[stephenquan]

Description of Change

Provide a fluent Bind extension method for BindableObject to support nested view models by adding BindingBase support to single value bindings (we already have BindingBase support for multi value bindings).

Linked Issues

• Fixes # Discussion [Proposal] Add BindingBase support for Bind extension methods to simplify nested bindings #416

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description) - not supplied (I think the unit test suffices)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pulls - We may need to rework https://learn.microsoft.com/en-us/dotnet/communitytoolkit/maui/markup/extensions/bindable-object-extensions#complex-nested-bindings

Additional information

[Copilot]

Pull request overview

Adds a fluent Bind extension overload that accepts a BindingBase, enabling direct use of BindingBase.Create(...) (including nested-property bindings) while preserving the existing fluent Markup style.

Changes:

• Add BindableObjectExtensions.Bind(..., BindingBase binding) convenience overload (wraps SetBinding).
• Add unit tests validating BindingBase.Create(...) works with nested properties and propagates updates for OneWay/TwoWay bindings.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/CommunityToolkit.Maui.Markup/BindableObjectExtensions.cs	Introduces a new Bind overload that applies a provided BindingBase to a target BindableProperty.
src/CommunityToolkit.Maui.Markup.UnitTests/BindableObjectExtensionsTests.cs	Adds coverage for the new overload using BindingBase.Create with nested property paths and change propagation.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[stephenquan]

@TheCodeTraveler I believe this is ready to review. Code is done. Unit test done. No sample provided, but, I think we can forego the sample because the unit test is sufficient. As for docs, I will need help since we need to determine how to message the handlers approach vs BindingBase.Create approach (or both).

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.