fix(settings): tolerate whitespace in header template delimiters (#148)

validateTemplateSecure matched safe functions with literal strings.Contains
("{{get "), so a space after {{ ("{{ get") failed the whitelist and
Config.Validate() rejected the config, surfacing as Traefik's opaque
"invalid handler type: <nil>". Go's text/template treats {{ get and {{get
identically, so the validator was stricter than the engine.

Normalize inner-delimiter whitespace before matching. This also closes a
security gap where a leading space let dangerous patterns ("{{ range",
"{{ call") evade detection.

Adds unit + regression coverage.

coverage_boost_final_test.go

@@ -750,6 +750,48 @@ func TestValidateTemplateSecure_CoverageBoost(t *testing.T) {
 			template:    "{{.Unknown}}",
 			shouldError: true,
 		},
+		// Issue #148: whitespace after {{ and before }} must be tolerated
+		{
+			name:        "get with leading space (issue #148)",
+			template:    "{{ get .Claims \"email\" }}",
+			shouldError: false,
+		},
+		{
+			name:        "get with multiple leading spaces",
+			template:    "{{  get .Claims \"email\"}}",
+			shouldError: false,
+		},
+		{
+			name:        "default with spaces",
+			template:    "{{ default \"unknown\" .Claims.email }}",
+			shouldError: false,
+		},
+		{
+			name:        "claims with spaces",
+			template:    "{{ .Claims.email }}",
+			shouldError: false,
+		},
+		{
+			name:        "access token with spaces",
+			template:    "{{ .AccessToken }}",
+			shouldError: false,
+		},
+		// Issue #148: space must NOT let dangerous patterns bypass detection
+		{
+			name:        "range with leading space stays blocked",
+			template:    "{{ range .Items }}{{.}}{{end}}",
+			shouldError: true,
+		},
+		{
+			name:        "call with leading space stays blocked",
+			template:    "{{ call .Func }}",
+			shouldError: true,
+		},
+		{
+			name:        "disallowed claim with spaces stays blocked",
+			template:    "{{ .Claims.password }}",
+			shouldError: true,
+		},
 	}
 
 	for _, tt := range tests {