fix: support 2.3.0 multi-arch image scan results

[ryanjpayne]

FCS CLI 2.3.x Multi-Architecture Support

Changes

• Auto-discover output files: Parse FCS CLI output for "Results saved to file:" messages instead of guessing file patterns
• Multi-arch support: Automatically handle multiple report files from multi-arch image scans
• SARIF conversion: Convert all architecture variants to SARIF when requested
• Fix output_path default: Remove "./" default that broke image scans; allow CLI defaults when omitted

Technical Details

• Modified execute_fcs_cli() to capture CLI output using tee while preserving exit codes
• Rewrote convert_json_to_sarif() to parse CLI output instead of using file pattern matching
• Updated action.yml to remove problematic default and improve description
• Added comprehensive documentation for FCS CLI 2.3.x multi-arch scanning

[carlosmmatos]

Pulled out the fallback path in convert_json_to_sarif. It only checked -d (directory), so file-based output_path values — which is the common image scan case — were silently skipped. The stdout-parsing path handles discovery fine on its own, so this was just dead weight.

Now it logs a warning if the CLI output file is missing. Also removed Tests 16-18 and the test_sarif_discovery_fallback helper since they tested the removed code. The 4 primary path tests still pass.

[carlosmmatos]

Waiting for GA then we can merge