Add --junitxml export to sigma check with bug fixes and tests

[Copilot]

Implements JUnit XML output for sigma check (from PR #83), fixing an UnboundLocalError identified in review and adding test coverage.

Changes

sigma/cli/check.py

• Add import xml.etree.ElementTree as ET, SEVERITY_ICONS dict, and generate_junit_report() — groups results into <testsuite> elements by error/issue type
• Add --junitxml <path> option; wire through check() and load_and_check_rules() via an optional junit_results list parameter
• Bug fix: In the SigmaConditionError handler, the original PR referenced rule_name/file_path variables that were never defined in that code path, causing an UnboundLocalError. Now reads directly from the current rule object:

  # Before (broken):
  "rule_name": rule_name,   # NameError — never assigned in this branch
  "file_path": file_path,
  # After:
  "rule_name": rule.title or str(rule.path),
  "file_path": str(rule.source) if rule.source else "unknown",

• Guard against None severity values with (res.get("severity") or "ok").lower() / (res.get("severity") or "UNKNOWN").upper()
• Generate the report after successful validation; also write an error-level entry when a top-level SigmaError is caught

tests/test_check.py

• test_check_junitxml_creates_file — verifies the XML file is created, well-formed, and rooted at <testsuites>
• test_check_junitxml_invalid — verifies failure entries are written for rules with parsing errors and that each testsuite/@failures count exactly matches the number of <failure> child elements

Both tests use tests/files/invalid with --pass-on-error to avoid the pre-existing network dependency on MITRE D3FEND data that already affects test_check_valid.