Add validation test for empty string sessionMetadataValue (MCP-3EG)

Added testGetRouteCoverage_SessionMetadataFilter_EmptyValue() to verify
that when sessionMetadataName is provided with a non-empty value but
sessionMetadataValue is an empty string, an IllegalArgumentException is
thrown with the message "sessionMetadataValue is required".

This completes the test coverage for MCP-3EG, which was already fixed
by the empty string handling changes in MCP-OU8 (commit d7eecba).

All 17 unit tests pass.

Fixes MCP-3EG

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

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

Author: ChrisEdwards

src/test/java/com/contrast/labs/ai/mcp/contrast/RouteCoverageServiceTest.java

@@ -286,6 +286,20 @@ void testGetRouteCoverage_SessionMetadataFilter_MissingValue() throws Exception
         verify(mockSDKExtension, never()).getRouteCoverage(anyString(), anyString(), any());
     }
 
+    @Test
+    void testGetRouteCoverage_SessionMetadataFilter_EmptyValue() throws Exception {
+        // Test validation with empty string for sessionMetadataValue (MCP-3EG)
+        // Act & Assert
+        IllegalArgumentException exception = assertThrows(IllegalArgumentException.class, () -> {
+            routeCoverageService.getRouteCoverage(TEST_APP_ID, TEST_METADATA_NAME, "", null);
+        });
+
+        assertTrue(exception.getMessage().contains("sessionMetadataValue is required"));
+
+        // Verify SDK was never called
+        verify(mockSDKExtension, never()).getRouteCoverage(anyString(), anyString(), any());
+    }
+
     // ========== Test Case 3: Latest Session Filter ==========
 
     @Test