Fix integration test: Add sourceFileFormat to test activity builder

ActivityControllerIntegrationTest was failing because the helper method
createTestActivity() didn't set the source_file_format field, which
became required after migration V15.

Added sourceFileFormat("FIT") to the Activity builder in the test helper.

Test Results:
- ✅ 128 tests passing
- ✅ 0 failures
- ✅ 0 errors
- 4 tests skipped (expected)

All integration tests now pass with Docker/Testcontainers.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/test/java/org/operaton/fitpub/controller/ActivityControllerIntegrationTest.java

@@ -304,6 +304,7 @@ class ActivityControllerIntegrationTest {
                 .totalDistance(BigDecimal.valueOf(5000))
                 .totalDurationSeconds(1800L)
                 .elevationGain(BigDecimal.valueOf(100))
+                .sourceFileFormat("FIT")
                 .build();
     }
 }