fix(core): preserve full path in workspace validation for non-existent segments

When realpathSync fails with ENOENT on non-existent paths, Node's error
object (e.path) only contains the path up to the last existing segment,
truncating the remaining portions. This causes incorrect path resolution
on UNC paths such as WSL (\wsl.localhost\...) where deeply nested
non-existent file paths lose their trailing segments.

Append the remaining non-existent segments from the original resolved
path to the prefix returned by realpathSync, ensuring the full intended
path is preserved for workspace boundary checks.

Fixes #18594

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: AnushkaPandit-21

packages/core/src/utils/workspaceContext.test.ts

@@ -162,6 +162,30 @@ describe('WorkspaceContext with real filesystem', () => {
       );
     });
 
+    it('should accept deeply nested non-existent paths within workspace', () => {
+      const workspaceContext = new WorkspaceContext(cwd);
+      const deepPath = path.join(
+        cwd,
+        'nonexistent',
+        'deeply',
+        'nested',
+        'file.txt',
+      );
+      expect(workspaceContext.isPathWithinWorkspace(deepPath)).toBe(true);
+    });
+
+    it('should reject deeply nested non-existent paths outside workspace', () => {
+      const workspaceContext = new WorkspaceContext(cwd);
+      const deepPath = path.join(
+        tempDir,
+        'outside',
+        'deeply',
+        'nested',
+        'file.txt',
+      );
+      expect(workspaceContext.isPathWithinWorkspace(deepPath)).toBe(false);
+    });
+
     describe.skipIf(os.platform() === 'win32')('with symbolic link', () => {
       describe('in the workspace', () => {
         let realDir: string;

packages/core/src/utils/workspaceContext.ts

@@ -227,8 +227,9 @@ export class WorkspaceContext {
    * if it did exist.
    */
   private fullyResolvedPath(pathToCheck: string): string {
+    const fullPath = path.resolve(this.targetDir, pathToCheck);
     try {
-      return fs.realpathSync(path.resolve(this.targetDir, pathToCheck));
+      return fs.realpathSync(fullPath);
     } catch (e: unknown) {
       if (
         isNodeError(e) &&
@@ -238,8 +239,16 @@ export class WorkspaceContext {
         // non-existent files.
         !this.isFileSymlink(e.path)
       ) {
-        // If it doesn't exist, e.path contains the fully resolved path.
-        return e.path;
+        // e.path only contains the path up to the last existing segment,
+        // which truncates the rest on UNC paths (e.g. WSL \\wsl.localhost\...).
+        // Append the remaining non-existent segments from the original path
+        // to preserve the full intended path.
+        const resolvedPrefix = e.path;
+        const remainder = fullPath.substring(resolvedPrefix.length);
+        if (remainder) {
+          return resolvedPrefix + remainder;
+        }
+        return resolvedPrefix;
       }
       throw e;
     }