feat(parsers): add Xygeni JSON parser (SAST, SCA, Secrets)

[lmrb-1968]

Description

This PR adds a single first-party parser for Xygeni JSON reports under
dojo/tools/xygeni/. It dispatches on metadata.scanType and exposes three
scan types: Xygeni SAST Scan, Xygeni SCA Scan, Xygeni Secrets Scan. The
pattern mirrors rusty_hog, anchore_grype, checkmarx, sonarqube, and
mobsf.

Xygeni is a Software Supply Chain Security platform that
emits a JSON report per scanner. The full pre-approval discussion (with
field-mapping tables and example JSON per kind) is at #14755.

Opened as a draft because pre-approval is still pending — happy to wait
for maintainer feedback before any further action. The implementation is
provided here so reviewers can evaluate the concrete shape if helpful.

Test results

unittests/tools/test_xygeni_parser.py covers:

• empty report per kind (3 fixtures)
• many-finding report per kind against real anonymized fixtures
  (501 SAST + 50 SCA + 61 Secrets findings)
• dispatch on metadata.scanType for a synthetic minimal report
• error paths: missing metadata.scanType, unsupported scan type

Run via the project's docker-compose unit-tests setup against real Postgres:

docker compose -f docker-compose.yml -f docker-compose.override.unit_tests.yml run --rm \
  --entrypoint /bin/bash uwsgi -lc \
  '. /secret-file-loader.sh; . /reach_database.sh; cd /app;
   unset DD_DATABASE_URL DD_CELERY_BROKER_URL;
   wait_for_database_to_be_reachable;
   python3 manage.py migrate --no-input;
   python3 manage.py test unittests.tools.test_xygeni_parser --keepdb -v 2'

Result: Ran 10 tests in 0.084s — OK. ruff check is clean against
dojo/tools/xygeni/ and unittests/tools/test_xygeni_parser.py.

Documentation

Added at docs/content/supported_tools/parsers/file/xygeni.md — covers all
three scan types, the common metadata envelope, the per-kind payload shapes,
and links to the sample fixtures.

Checklist

• Rebased on the latest dev.
• Targets dev (new parser).
• Ruff compliant (line-length 120, target py313).
• Python 3.13 compliant.
• Documentation page added.
• No model changes — no migration needed.
• Unit tests added.
• Label Import Scans requested — cannot self-apply as a non-collaborator.

---

Pre-approval: #14755

[valentijnscholten]

At first glance it looks good, but shouldn't settings.dist.py not be updated to set the dedupe algorithm to UNIQUE_ID_FROM_TOOL (or UNIQUE_ID_FROM_TOOL_OR_HASH)?