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>

example/go.mod

@@ -2,4 +2,4 @@ module github.com/imjasonh/govulncheck-action/example
 
 go 1.20
 
-require golang.org/x/net v0.0.0-20220906165146-f3363e06e74c
+require golang.org/x/net v0.22.0