Report the failing nested response file

[zjdangge]

Nested response-file read failures are currently reported against the outermost response file. Track the path at each file read so that missing-file and other I/O errors identify the file that actually failed.

Adds regression tests for missing files, missing directories, and locked files at two nesting depths. Existing path resolution and the handling of partially expanded tokens are preserved.

Fixes #2324.

Validation on Windows:

• All six new regression cases fail before the fix and pass afterward.
• Debug solution build: no warnings or errors.
• All five Debug test assemblies: 1,953 passed, 0 failed, 14 skipped by existing test configuration, including the core tests on .NET 10 and .NET Framework 4.7.2.

Tests ran with an English UI culture and xUnit AppDomains disabled. The build/test scripts and four existing tests have issues with paths containing spaces, so the built test assemblies were run with VSTest from a temporary directory without spaces.