Rename OnFailureActivity to OnSignInFailureActivity

[corinagum]

To avoid the risk of future namespace collisions, rename OnFailureActivity to OnSignInFailureActivity.

• Renamed the protected default handler OnFailureActivity to OnSignInFailureActivity to match naming convention & reduce risk of naming collisions
• Align with updated messaging across SDKs to update sign-in failure sample string

[Copilot]

Pull request overview

This PR renames the built-in sign-in failure invoke handler to a more specific name to reduce the chance of future naming/namespace collisions, and updates the Graph sample’s sign-in failure user message to match the revised wording.

Changes:

• Renamed the default handler method from OnFailureActivity to OnSignInFailureActivity.
• Updated the app constructor wiring to register the renamed handler for OnSignInFailure(...).
• Updated the Graph sample’s sign-in failure message text.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
Samples/Samples.Graph/Program.cs	Updates the user-facing sign-in failure message string in the sample.
Libraries/Microsoft.Teams.Apps/AppRouting.cs	Renames the protected default signin/failure invoke handler method.
Libraries/Microsoft.Teams.Apps/App.cs	Updates default handler registration to use the renamed method.

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