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.

cc @dotnet/appmodel @AaronRobinsonMSFT

Changes Made

Core Logic Enhancement:

• 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 message for the above

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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