fix: resolve ReportParsingError in IaC mode

[tembleking]

Summary

IaC mode scans were silently failing with ReportParsingError because the scanner always attempted to parse a JSON output file that only exists in VM mode.

Root Cause

In VM mode, the Sysdig CLI scanner generates a scan-result.json file via the --output=json-file=scan-result.json flag. The action then reads and parses this file to build the ScanResult object.

In IaC mode, this flag is intentionally omitted (IaC scans don't produce vulnerability JSON), but the code still attempted to read and parse the non-existent file, causing JSON.parse("") to throw.

Why This Wasn't Caught Earlier

The CI dogfooding test (scan-with-multiple-policies) was running with stop-on-processing-error: false (the default). This meant the ReportParsingError was logged but didn't fail the action, so steps.scan.outcome was "success" and the test passed despite the underlying error.

Changes

• IaC mode now returns a minimal ScanResult derived from the CLI exit code (0 = passed, non-zero = failed) instead of attempting to parse JSON
• The CI test now uses stop-on-processing-error: true to ensure processing errors are caught in the future

Closes #99