Add searched .NET install locations to error message

[elinor-fung]

Include searched locations in error message when host cannot find a .NET install (hostfxr)

Example - the 'following locations were searched' section is new:

You must install .NET to run this application.

App: /home/elinor/helloworld/bin/Debug/net10.0/helloworld
Architecture: x64
App host version: 10.0.0-dev
.NET location: Not found

The following locations were searched:
  Application directory:
    /home/elinor/helloworld/bin/Debug/net10.0/
  Environment variable:
    DOTNET_ROOT_X64 = <not set>
    DOTNET_ROOT = <not set>
  Registered location:
    /etc/dotnet/install_location_x64 = <not set>
  Default location:
    /usr/share/dotnet

Learn more:
https://aka.ms/dotnet/app-launch-failed

Download the .NET runtime:
https://aka.ms/dotnet-core-applaunch?missing_runtime=true&arch=x64&rid=linux-x64&os=ubuntu.22.04&apphost_version=10.0.0-dev

Resolves #116108

cc @dotnet/appmodel @AaronRobinsonMSFT

[Copilot]

Pull Request Overview

This PR enhances the error messaging when the .NET host cannot find a .NET installation by adding detailed information about which locations were searched during the discovery process.

Key changes:

• Modified get_dotnet_root_from_env to clear the environment variable name when no valid path is found
• Enhanced error message generation in fxr_resolver.cpp to include a comprehensive list of searched locations
• Added a new test case to verify the improved error message format

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/native/corehost/hostmisc/utils.cpp	Modified to clear environment variable name when dotnet root is not found
src/native/corehost/fxr_resolver.cpp	Enhanced error message generation with detailed search location information
src/native/corehost/corehost.cpp	Removed redundant error logging since detailed error is now handled elsewhere
src/native/corehost/apphost/standalone/hostfxr_resolver.cpp	Added error message for non-fully-qualified hostfxr paths
src/installer/tests/HostActivation.Tests/InstallLocation.cs	Added test case to verify new error message format

Comments suppressed due to low confidence (1)

src/native/corehost/fxr_resolver.cpp:162

• [nitpick] The variable name 'searched_locations' could be more descriptive. Consider renaming it to 'search_details_message' or 'location_search_summary' to better indicate it's building an error message.

    pal::string_t searched_locations = _X("The following locations were searched:");

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[AaronRobinsonMSFT]

Minor style nit.

[elinor-fung]

/ba-g #117811 and timeout in leg unaffected by this change (libraries tests linux_musl)