Fix several edge cases when the workspace folder is a filesystem root

[vanklompf]

Background

microsoft/vscode#317637 fixes a bug in the built-in git extension where a custom normalizePath() helper unconditionally stripped trailing path separators, turning the filesystem root / (e.g. in a chroot) into "." and breaking descendant / equality / relative-path checks. While reviewing this repo for the same pattern I found that the exact normalizePath bug is not present here, but several other code paths misbehave when the workspace folder is a filesystem root (/ on POSIX, drive root like C:\ on Windows).

This PR fixes three such issues. They are split into three logical commits.

1. onSwitchHeaderSource substring-based path strip

The old code rewrote the symlink-resolved target filename via:

targetFileName = client.RootPath + targetFileName.substring(client.RootRealPath.length);

If RootRealPath is a filesystem root (e.g. "/"), substring("/".length) drops the leading separator, producing a malformed result such as "/symlinkfoo.h" when RootPath = "/symlink" and targetFileName = "/foo.h". The descendant check via startsWith(RootRealPath) also matches unrelated siblings like /foo vs /foobar.

Replaced with path.relative + path.join, which handles filesystem roots correctly and avoids the false prefix match. This is the spirit of the fix in microsoft/vscode#317637.

2. rootUri.fsPath + path.sep + … concatenations

CppProperties had four sites that built paths via:

rootUri.fsPath + path.sep + something

For a workspace at /, this yields "//something". POSIX tolerates the double slash, but it surfaces verbatim in user-visible diagnostics like Cannot find: //usr/include/foo. On Windows with workspace C:\, the result is similarly malformed.

Replaced with path.join. checkPathExistsSync is updated to use path.join internally too; its second parameter is renamed from relativePath to baseDir and no longer requires a trailing separator. All callers updated.

3. ${workspaceFolderBasename} for root workspaces

path.basename("/") and path.basename("C:\\") both return "". This caused two related problems:

• expand.ts treats a defined-but-empty variable value the same as an undefined variable (if (!repl)) and emits an Invalid variable reference ${workspaceFolderBasename} warning while leaving the literal ${...} in the output. Changed to distinguish undefined from "".
• The two sites that compute workspaceFolderBasename (CppProperties.ExtendedEnvironment and DefaultClient.AdditionalEnvironment) disagreed (path.basename(fsPath) vs workspaceFolder.name) and both could yield empty values at root. Both now use path.basename(fsPath) || fsPath so the substitution is never empty for a defined workspace folder. The debugger's ${workspaceFolderBasename} substitution gets the same fallback.

Testing

• yarn build (full TypeScript build) passes.
• eslint on all changed files passes with no warnings.
• No automated tests existed for these specific paths; the changes are minimal and preserve behavior for non-root workspaces.

Notes

• These are not security or correctness bugs in the everyday case — they only manifest when the workspace folder is the filesystem root (chroot, drive root) or, for fix Fix several edge cases when the workspace folder is a filesystem root #1, when the workspace folder is a symlink that resolves to the filesystem root.
• No public API surface changes. The only signature change is checkPathExistsSync(inputPath, baseDir, …) (renamed parameters), a non-exported convenience helper used only within this extension.