fileBasedApps: unit tests, no-csproj cone detection, language-ID check

[Copilot]

Improves the "Convert to Project" command's FBA entry-point discovery with three behavioural changes and adds unit test coverage for the previously untestable detection logic.

Changes

Testability refactor (convertToProject.ts)

• const enum FileBasedAppKind → export enum; detectFileBasedAppKind now exported with an injectable readFileHead parameter (defaults to the real fs impl), eliminating the need to mock fs in tests.
• Real file-reading logic extracted into private defaultReadFileHead.

No-.csproj-cone detection (item 2)

• New exported isInProjectCone(filePath, csprojDirs) walks ancestor directories to check for a .csproj.
• pickAndConvertToProject now collects all .csproj locations via findFiles, then includes any .cs file not inside any project cone in the picker — even without #!/#: markers — since such files have no project to belong to.

VS Code language ID instead of file extension (item 3)

• Right-click path: opens the document (or finds it if already open) and checks document.languageId === 'csharp' instead of filePath.endsWith('.cs').
• Palette path: supplements findFiles('**/*.cs') with any already-open textDocuments whose languageId is csharp but whose path wasn't caught by the glob.

SDK detection investigation (item 4)

dotnet project in .NET 10 only exposes convert (no dry-run/detect), and dotnet run --file actually runs the program. No viable CLI detection path exists today. A TODO in pickAndConvertToProject records this and flags workspace/_ms_fileBasedProgramEntryPoints as the right long-term replacement once Roslyn exposes it.

Unit tests (test/lsptoolshost/unitTests/convertToProject.test.ts)

20 new tests across detectFileBasedAppKind and isInProjectCone:

// No fs mocking needed — tests inject a string reader directly
test('returns Shebang for a file starting with #!', () => {
    expect(detectFileBasedAppKind('/a/app.cs', makeReader('#!/usr/bin/env dotnet\n...'))).toBe(FileBasedAppKind.Shebang);
});

test('returns true when the file is in a subdirectory of a csproj directory', () => {
    const csprojDirs = new Set(['/workspace/project']);
    expect(isInProjectCone('/workspace/project/src/Foo.cs', csprojDirs)).toBe(true);
});

Covers: shebang, #: directives, BOM handling, unreadable files, 5-line scan cutoff, and all isInProjectCone ancestor/sibling permutations.