feat(sca): add unresolved reason and errors to subproject stats (#356)

In semgrep/semgrep-proprietary#2872, we updated
the SCA subproject logic to scan only changed subprojects in diff scans.
Subprojects that remained unchanged were intentionally left unresolved
with the reason UnresolvedSkipped.

Previously, we didn't include this information in our subproject stats,
leaving us without visibility into why a subproject was unresolved. This
PR introduces two optional fields, unresolved_reason and errors,
allowing us to capture more granular stats and better understand
subproject resolution failures.

- [x] I ran `make setup && make` to update the generated code after
editing a `.atd` file (TODO: have a CI check)
- [x] I made sure we're still backward compatible with old versions of
the CLI.
For example, the Semgrep backend need to still be able to *consume* data
	  generated by Semgrep 1.50.0.
See
https://atd.readthedocs.io/en/latest/atdgen-tutorial.html#smooth-protocol-upgrades
	  Note that the types related to the semgrep-core JSON output or the
	  semgrep-core RPC do not need to be backward compatible!

Author: salolivares

semgrep_output_v1.atd

@@ -1730,6 +1730,10 @@ type subproject_stats = {
   dependency_sources: dependency_source_file list;
   (* Results of dependency resolution, empty if resolution failed *)
   ?resolved_stats: dependency_resolution_stats option;
+  (* Reason why resolution failed, empty if resolution succeeded *)
+  ?unresolved_reason: unresolved_reason option;
+  (* Errors encountered during subproject resolution *)
+  ~errors: sca_error list
 }
 
 type dependency_source_file = {