File-based apps: support #:include .dll by default

[jjonescz]

No description provided.

[Copilot]

Pull request overview

This PR updates file-based apps support so that #:include directives targeting .dll files work out-of-the-box by mapping .dll to the MSBuild Reference item type in the default include/exclude mapping.

Changes:

• Extend the default FileBasedProgramsItemMapping to include .dll=Reference.
• Update run-api expected virtual project output to reflect the new default mapping.
• Simplify dotnet project convert tests by removing now-unnecessary .dll mapping directives, and adjust one test to explicitly clear the mapping to preserve the “no mapping” scenario.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
test/dotnet.Tests/CommandTests/Run/RunFileTests_CscOnlyAndApi.cs	Updates expected run-api virtual project XML to include .dll=Reference in the mapping.
test/dotnet.Tests/CommandTests/Project/Convert/DotnetProjectConvertTests.cs	Removes explicit .dll mapping setup in tests that now rely on the default mapping; adds an explicit empty mapping for the “no mapping” case.
src/Cli/Microsoft.DotNet.FileBasedPrograms/FileLevelDirectiveHelpers.cs	Adds .dll=Reference to the default mapping and adjusts mapping parsing behavior.

[jjonescz]

@333fred @RikkiGibson for reviews, thanks