refactor: extract repeated string literals into constants

- Add tokenParam const in jenkins.go and reuse across main.go
- Add shared test_helpers_test.go with test constants
- Remove unused //nolint:gosec directive in jenkins.go
- Resolve golangci-lint v2.12 goconst and nolintlint warnings

test_helpers_test.go

@@ -0,0 +1,18 @@
+package main
+
+const (
+	testUserFoo          = "foo"
+	testUserBar          = "bar"
+	testUserName         = "test"
+	testJobName          = "test-job"
+	testExampleURL       = "http://example.com"
+	testAuthRequiredErr  = "authentication required"
+	testRemoteTokenValue = "remote-token-123"
+	testWhitespaceVal    = "   "
+	testValidStr         = "valid"
+	testParamKey         = "key"
+	testParamKey1        = "key1"
+	testParamKey2        = "key2"
+	testParamValue1      = "value1"
+	testParamValue2      = "value2"
+)