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

[Copilot]

Currently, when a user runs dotnet <app_path> and the host determines that <app_path> is not a runnable app, it's treated as a CLI command and routed to the SDK, which shows a generic error message.

This PR improves the user experience by detecting when the user passes a path to an existing non-dll/exe file and showing a specific error message instead of routing to the SDK.

Changes Made

Core Logic Enhancement:

• Added new status code AppArgNotManagedFailure (0x80008090) using an unused error code slot
• Enhanced argument parsing in command_line.cpp to detect the specific scenario:
  • File is not managed (.dll/.exe)
  • Path contains directory separator
  • File exists
  • Not in exec mode
• Added specific error handling in fx_muxer.cpp with clear error message

Behavior Change:

Before:

$ dotnet /path/to/config.json
Could not execute because the specified command or file was not found.
Possible reasons for this include:
  You misspelled a built-in dotnet command.
  You intended to execute a .NET program, but dotnet-/path/to/config.json does not exist.
  You intended to run a global tool, but a dotnet-prefixed executable with this name could not be found on the PATH.

After:

$ dotnet /path/to/config.json
The application '/path/to/config.json' does not exist or is not a managed .dll or .exe.

Preserves Existing Behavior:

• Commands without directory separators (like dotnet build) still route to SDK
• dotnet exec behavior unchanged
• All existing error codes and messages unchanged

Test Coverage

Added comprehensive tests covering:

• Absolute paths with directory separators
• Relative paths like ./subdir/file.txt
• Commands without separators still route to SDK
• Backward compatibility verification

All existing tests pass, confirming no regressions.

Fixes #116939.

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[jkotas]

Suggested change

	trace::error(_X("The application '%s' does not exist or is not a managed .dll or .exe."), app_candidate.c_str());
	trace::error(_X("The application '%s' is not a managed .dll or .exe."), app_candidate.c_str());

This error code is returned above only when the file actually exists

[Copilot]

Updated the error message to remove "does not exist or" since this error code is only returned when the file actually exists. Changes committed in d2dd638.

[elinor-fung]

Suggested change

	string relativePath = "./subdir/test.json";
	string relativePath = Path.GetRelativePath(sharedTestState.BaseDirectory.Location, testFile);

[Copilot]

Changed to use Path.GetRelativePath(sharedTestState.BaseDirectory.Location, testFile) for dynamic path computation. This makes the test more robust and maintainable. Changes committed in df74861.