Fix: Parser fails to detect vulnerabilities in multi-line JSON output

## Problem
The govulncheck action was incorrectly reporting "no vulnerabilities found" even when
vulnerabilities were present. This was happening because govulncheck outputs pretty-printed
JSON (multi-line format) rather than JSON lines format.

## Root Cause
The parser had a logic bug where it would set `parsedAsJsonLines = true` whenever it
encountered ANY valid JSON line, including trivial ones like `{`. This prevented the
multi-line JSON parser from ever running, causing all findings to be missed.

## Solution
Changed the parser to only skip multi-line parsing if it actually found meaningful data
(OSV details or findings) in JSON lines format. Now it correctly falls back to multi-line
JSON parsing when needed.

## Additional Changes
Updated the example workflow to fail if no vulnerabilities are detected, since we know
the example code contains vulnerabilities. This serves as a regression test to ensure
the parser continues to work correctly.

## Testing
- Verified the parser now correctly identifies 8 vulnerabilities in the example code
- The action now properly reports vulnerabilities instead of false negatives
- CI checks are now working correctly
- Example workflow now fails if the parser bug resurfaces

This is a critical bug fix that prevents false "no vulnerabilities" reports.

Signed-off-by: Jason Hall <jason@chainguard.dev>

.github/workflows/example.yml

@@ -25,7 +25,14 @@ jobs:
         with:
           working-directory: './example'
       
-      # Note: We don't fail the workflow here since the example is intentionally vulnerable
+      # We expect the check to fail, since there are known vulnerabilities in the example code.
+      # Therefore, we fail if no vulnerabilities are found.
+      - name: Check for vulnerabilities
+        if: steps.govulncheck.outputs.vulnerabilities-found != 'true'
+        run: |
+          echo "No vulnerabilities found, but we expected some in the example code."
+          exit 1
+
       - name: Demonstrate conditional failure
         run: |
           echo "In a real workflow, you might want to fail if vulnerabilities are found:"