feat: add skipped_paths field to ci_scan_results (#427)

## Summary
Add optional `skipped_paths` field to the `ci_scan_results` type to
communicate which files were detected but couldn't be scanned due to
errors (timeout, OOM, stack overflow, etc.).

This allows the app to avoid marking findings in these files as "fixed"
when they weren't actually scanned.

## Changes
- Added `?skipped_paths` field to `ci_scan_results` in
`semgrep_output_v1.atd`
- Regenerated all derived files (Python, OCaml, JSON Schema, Proto, ...)

## Backwards Compatibility
- Field is optional (`?` prefix), so existing consumers will still work
- Field is omitted from JSON when None/empty

---

- [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!
- [x] Any accompanying changes in `semgrep-proprietary` are approved and
ready to merge once this PR is merged

Author: squaresurf

semgrep_output_v1.atd

@@ -2031,8 +2031,11 @@ type ci_scan_results = {
    (* TODO? use a token type ? *)
    token: string nullable;
 
-   searched_paths: fpath list;
+   searched_paths
+     <doc text="Files that were detected and attempted to scan. Note that some of these may have been skipped due to errors (see skipped_paths).">
+   : fpath list;
    renamed_paths: fpath list;
+   ~skipped_paths <doc text="Files detected but not scanned due to errors (timeout, OOM, etc.). The app should NOT mark findings in these files as fixed.">: fpath list;
 
    rule_ids: rule_id list;