Migrate IFileSystem to TestableIO System.IO.Abstractions

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### Description

Replace the custom `IFileSystem` abstraction in `src/Func/Common/` with [TestableIO System.IO.Abstractions](https://www.nuget.org/packages/TestableIO.System.IO.Abstractions) and its test helpers package. Benefits:

- Standardizes on the same abstraction used in other Azure Functions projects
- Eliminates the need to test our own `PhysicalFileSystem` implementation
- Leverages TestableIO's `MockFileSystem` in tests instead of hand-rolled fakes

The custom `IFileSystem` currently has methods not covered by TestableIO (archive extraction, SHA-256 hashing). These should move to a separate `IArchiveService` or similar interface.

Consumers to migrate: `CliUpdater`, `UserProfileSource`, `ProjectProfileSource`, `ProjectProfileConfigStore`, `RemoteProfileSource`, and their tests.

Start by reviewing the custom IFileSystem abstraction in src/Func/Common/ and the listed consumers: CliUpdater, UserProfileSource, ProjectProfileSource, ProjectProfileConfigStore, and RemoteProfileSource. Compare their tests with TestableIO System.IO.Abstractions and its test helpers, then migrate the consumers and tests so MockFileSystem replaces hand-rolled fakes and archive extraction and SHA-256 hashing have a separate service interface.

Written by the indexing model from the issue text.