Autoinclude Error Reporting

stack-dependencies — Stricter validation and clearer parse errors for autoinclude

Malformed configuration inside an autoinclude block previously produced misleading messages or, in some cases, was silently ignored during stack discovery.

Two changes tighten this up:

• A dependency block inside autoinclude must declare exactly one label. Zero labels (dependency {}) and multiple labels (dependency "a" "b" {}) are now rejected at parse time with a diagnostic that points at the offending block.
• Parse failures encountered while expanding autoinclude files during stack generate, find, and list are surfaced with the underlying HCL diagnostic instead of being swallowed or remapped to a generic discovery error.

Reported in #5980.