Refactor context flash msg and global variables (#33375)

1. add `GetSiteCookieFlashMessage` to help to parse flash message
2. clarify `handleRepoHomeFeed` logic
3. remove unnecessary global variables, use `sync.OnceValue` instead
4. add some tests for `IsUsableUsername` and `IsUsableRepoName`

modules/validation/regex_pattern_test.go

@@ -17,40 +17,40 @@ func getRegexPatternErrorString(pattern string) string {
 	return ""
 }
 
-var regexValidationTestCases = []validationTestCase{
-	{
-		description: "Empty regex pattern",
-		data: TestForm{
-			RegexPattern: "",
-		},
-		expectedErrors: binding.Errors{},
-	},
-	{
-		description: "Valid regex",
-		data: TestForm{
-			RegexPattern: `(\d{1,3})+`,
-		},
-		expectedErrors: binding.Errors{},
-	},
-
-	{
-		description: "Invalid regex",
-		data: TestForm{
-			RegexPattern: "[a-",
-		},
-		expectedErrors: binding.Errors{
-			binding.Error{
-				FieldNames:     []string{"RegexPattern"},
-				Classification: ErrRegexPattern,
-				Message:        getRegexPatternErrorString("[a-"),
-			},
-		},
-	},
-}
-
 func Test_RegexPatternValidation(t *testing.T) {
 	AddBindingRules()
 
+	regexValidationTestCases := []validationTestCase{
+		{
+			description: "Empty regex pattern",
+			data: TestForm{
+				RegexPattern: "",
+			},
+			expectedErrors: binding.Errors{},
+		},
+		{
+			description: "Valid regex",
+			data: TestForm{
+				RegexPattern: `(\d{1,3})+`,
+			},
+			expectedErrors: binding.Errors{},
+		},
+
+		{
+			description: "Invalid regex",
+			data: TestForm{
+				RegexPattern: "[a-",
+			},
+			expectedErrors: binding.Errors{
+				binding.Error{
+					FieldNames:     []string{"RegexPattern"},
+					Classification: ErrRegexPattern,
+					Message:        getRegexPatternErrorString("[a-"),
+				},
+			},
+		},
+	}
+
 	for _, testCase := range regexValidationTestCases {
 		t.Run(testCase.description, func(t *testing.T) {
 			performValidationTest(t, testCase)