Fix azure.IsNotFound for new azcore error type

[thomas11]

Fixes an issue I discovered today when trying to reproduce another issue report. pulumi refresh is broken for this provider:

$ PATH="$HOME/pulumi/pan/bin:$PATH" pulumi refresh --skip-preview -s dev

     pulumi:pulumi:Stack                      recoveryservices-protecteditem-dev  **failed**                1 error; 4 warnings
 ~   ├─ azure-native:resources:ResourceGroup  resourceGroup                       **refreshing failed**     1 error

  azure-native:resources:ResourceGroup (resourceGroup):
    error: Code="ResourceGroupNotFound" Message="Resource group 'resourceGroup3f871416' could not be found."

The reason is that #3783 regressed azure.IsNotFound, causing Read to report an error when a resource doesn't exist in Azure.

This PR fixes the problem and adds test coverage.

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

[pulumi-bot]

This PR has been shipped in release v2.82.0.

[EronWright]

I have to wonder if using errors.As would have helped here. That is, wrap the underlying error to improve the formatting, and have client code use As consistently.