feat [task-04]: Update GCC test to include severity field

The generic SARIF parser now extracts severity from the level field.
This affects all SARIF-based analyzers including GCC. Updated the
expected plist to include severity=MEDIUM (from SARIF level=warning).

Verification results for seqra.sarif.json:
- 144 reports processed successfully
- 71 HIGH severity, 73 MEDIUM severity
- 11 different CWE tags extracted
- All analyzer unit tests pass (89/89)

Author: seqradev

tools/report-converter/tests/unit/analyzers/gcc_output_test_files/double_free.expected.plist

@@ -233,6 +233,8 @@
 					</array>
 				</dict>
 			</array>
+			<key>severity</key>
+			<string>MEDIUM</string>
 			<key>type</key>
 			<string>gcc</string>
 		</dict>