fix(gin): reset skippedNodes between method tree probes in 405 handling

handleHTTPRequest reused c.skippedNodes across the per-method getValue
probes that build the Allow header when HandleMethodNotAllowed is
enabled. A probe that matched via a static branch left its skipped
wildcard entries on the stack, and when a later tree's probe
dead-ended it could roll back through such a stale entry into the
previous tree's nodes, find a handler there, and wrongly list its own
method in the Allow header. Reset the stack before each probe.

gin.go

@@ -743,6 +743,7 @@ func (engine *Engine) handleHTTPRequest(c *Context) {
 			if tree.method == httpMethod {
 				continue
 			}
+			*c.skippedNodes = (*c.skippedNodes)[:0]
 			if value := tree.root.getValue(rPath, nil, c.skippedNodes, unescape); value.handlers != nil {
 				allowed = append(allowed, tree.method)
 			}

routes_test.go

@@ -527,6 +527,23 @@ func TestRouteNotAllowedEnabled3(t *testing.T) {
 	assert.Contains(t, allowed, http.MethodPost)
 }
 
+func TestRouteNotAllowedDoesNotReuseSkippedNodes(t *testing.T) {
+	router := New()
+	router.HandleMethodNotAllowed = true
+	router.POST("/b", func(c *Context) {})
+	router.POST("/:p0", func(c *Context) {})
+	router.PUT("/a/:p1", func(c *Context) {})
+
+	// The POST probe for the Allow header matches static "/b" while
+	// recording the skipped wildcard ":p0". handleHTTPRequest must reset
+	// c.skippedNodes before probing the PUT tree, or the stale entry
+	// leaks the POST route into the PUT lookup and PUT is wrongly
+	// reported in the Allow header.
+	w := PerformRequest(router, http.MethodGet, "/b")
+	assert.Equal(t, http.StatusMethodNotAllowed, w.Code)
+	assert.Equal(t, http.MethodPost, w.Header().Get("Allow"))
+}
+
 func TestRouteNotAllowedDisabled(t *testing.T) {
 	router := New()
 	router.HandleMethodNotAllowed = false