Revert "Show specific error message for dotnet <app_path> where app_path is a non-dll/exe file that exists"

[elinor-fung]

Reverts #116940

See #116940 (comment)

[Copilot]

Pull Request Overview

This PR reverts a previous change that added specific error messaging for when users run dotnet <app_path> where the app_path is an existing non-dll/exe file. The revert removes the special case handling and returns to the original behavior where such cases are routed to the CLI instead of showing a specific error.

• Removes the special case detection for non-managed files with directory separators
• Reverts error message back to the generic "does not exist or is not a managed .dll or .exe" message
• Removes associated test cases that verified the specific error behavior

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/native/corehost/fxr/command_line.cpp	Removes the logic that detects existing non-managed files with directory separators and shows specific error messages
src/installer/tests/HostActivation.Tests/FrameworkDependentAppLaunch.cs	Updates test assertion to expect the generic error message instead of the specific one
src/installer/tests/HostActivation.Tests/DotnetArgValidation.cs	Removes two test methods that verified the specific error behavior for non-managed files

[dotnet-policy-service]

Tagging subscribers to this area: @vitek-karas, @agocke, @VSadov
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[agocke]

LGTM. Sounds like this should be an SDK feature instead.

[jjonescz]

/backport to release/10.0-preview7

[github-actions]

Started backporting to release/10.0-preview7: https://github.com/dotnet/runtime/actions/runs/16528196413