Add edge case tests for file extension validation

Added test cases for edge scenarios:
- Files with multiple dots (my.config.yaml, config.backup.txt)
- Hidden files with extensions (.config.yaml, .config.txt)
- Files with only dot and extension (.yaml, .yml)

Total test cases increased from 10 to 19, improving coverage for
various filename patterns that users might encounter.

Co-authored-by: dlevy-msft-sql <[email protected]>

Author: Copilot

internal/config/viper_test.go

@@ -70,6 +70,51 @@ func Test_validateConfigFileExtension(t *testing.T) {
 			filename: "config.Yaml",
 			wantErr:  false,
 		},
+		{
+			name:     "multiple dots with valid extension",
+			filename: "my.config.yaml",
+			wantErr:  false,
+		},
+		{
+			name:     "multiple dots with invalid extension",
+			filename: "my.config.txt",
+			wantErr:  true,
+		},
+		{
+			name:     "backup file with valid extension",
+			filename: "config.backup.yaml",
+			wantErr:  false,
+		},
+		{
+			name:     "backup file with invalid extension",
+			filename: "config.backup.txt",
+			wantErr:  true,
+		},
+		{
+			name:     "hidden file with yaml extension",
+			filename: ".config.yaml",
+			wantErr:  false,
+		},
+		{
+			name:     "hidden file with yml extension",
+			filename: ".config.yml",
+			wantErr:  false,
+		},
+		{
+			name:     "hidden file with invalid extension",
+			filename: ".config.txt",
+			wantErr:  true,
+		},
+		{
+			name:     "file with only dot and yaml",
+			filename: ".yaml",
+			wantErr:  false,
+		},
+		{
+			name:     "file with only dot and yml",
+			filename: ".yml",
+			wantErr:  false,
+		},
 	}
 
 	for _, tt := range tests {