Add BindingContext to Behaviors using Bindings (.NET 9)

[brminnick]

This PR adds a BindingContext to each Behavior sample code that uses bindings.

In the .NET 9 release, we are no longer automatically setting the BindingContext for our Behaviors. We notified developers in the docs on April 2nd of this change (screenshot below) and encouraged all of them to manually set a BindingContext:

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Learn Build status updates of commit 9caf5fd:

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docs/maui/behaviors/maximum-length-reached-behavior.md	✅Succeeded	View
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[learn-build-service-prod]

Learn Build status updates of commit 1694bfa:

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docs/maui/behaviors/maximum-length-reached-behavior.md	✅Succeeded	View
docs/maui/behaviors/touch-behavior.md	✅Succeeded	View
docs/maui/behaviors/user-stopped-typing-behavior.md	✅Succeeded	View

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[learn-build-service-prod]

Learn Build status updates of commit 0e8a470:

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docs/maui/behaviors/event-to-command-behavior.md	✅Succeeded	View
docs/maui/behaviors/image-touch-behavior.md	✅Succeeded	View
docs/maui/behaviors/maximum-length-reached-behavior.md	✅Succeeded	View
docs/maui/behaviors/progressbar-animation-behavior.md	✅Succeeded	View
docs/maui/behaviors/touch-behavior.md	✅Succeeded	View
docs/maui/behaviors/user-stopped-typing-behavior.md	✅Succeeded	View

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[learn-build-service-prod]

Learn Build status updates of commit f41d584:

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docs/maui/behaviors/animation-behavior.md	✅Succeeded	View
docs/maui/behaviors/event-to-command-behavior.md	✅Succeeded	View
docs/maui/behaviors/image-touch-behavior.md	✅Succeeded	View
docs/maui/behaviors/maximum-length-reached-behavior.md	✅Succeeded	View
docs/maui/behaviors/progressbar-animation-behavior.md	✅Succeeded	View
docs/maui/behaviors/touch-behavior.md	✅Succeeded	View
docs/maui/behaviors/user-stopped-typing-behavior.md	✅Succeeded	View

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