feat(rules): replace go templates with custom variable expansion

- Replace template syntax ({{ .Request.Method }}) with $-prefixed variables ($req_method)
- Implement custom variable parser with static ($req_method, $status_code) and dynamic ($header(), $arg(), $form()) variables
- Replace templateOrStr interface with templateString struct and ExpandVars methods
- Add parser improvements for reliable quote handling
- Add new error types: ErrUnterminatedParenthesis, ErrUnexpectedVar, ErrExpectOneOrTwoArgs
- Update all tests and help text to use new variable syntax
- Add comprehensive unit and benchmark tests for variable expansion

internal/route/rules/vars_static.go

@@ -0,0 +1,92 @@
+package rules
+
+import (
+	"net"
+	"net/http"
+	"strconv"
+
+	"github.com/yusing/godoxy/internal/route/routes"
+)
+
+const (
+	VarRequestMethod      = "req_method"
+	VarRequestScheme      = "req_scheme"
+	VarRequestHost        = "req_host"
+	VarRequestPort        = "req_port"
+	VarRequestPath        = "req_path"
+	VarRequestAddr        = "req_addr"
+	VarRequestQuery       = "req_query"
+	VarRequestURL         = "req_url"
+	VarRequestURI         = "req_uri"
+	VarRequestContentType = "req_content_type"
+	VarRequestContentLen  = "req_content_length"
+	VarRemoteHost         = "remote_host"
+	VarRemotePort         = "remote_port"
+	VarRemoteAddr         = "remote_addr"
+
+	VarUpstreamName   = "upstream_name"
+	VarUpstreamScheme = "upstream_scheme"
+	VarUpstreamHost   = "upstream_host"
+	VarUpstreamPort   = "upstream_port"
+	VarUpstreamAddr   = "upstream_addr"
+	VarUpstreamURL    = "upstream_url"
+
+	VarRespContentType = "resp_content_type"
+	VarRespContentLen  = "resp_content_length"
+	VarRespStatusCode  = "status_code"
+)
+
+var staticReqVarSubsMap = map[string]reqVarGetter{
+	VarRequestMethod: func(req *http.Request) string { return req.Method },
+	VarRequestScheme: func(req *http.Request) string {
+		if req.TLS != nil {
+			return "https"
+		}
+		return "http"
+	},
+	VarRequestHost: func(req *http.Request) string {
+		reqHost, _, err := net.SplitHostPort(req.Host)
+		if err != nil {
+			return req.Host
+		}
+		return reqHost
+	},
+	VarRequestPort: func(req *http.Request) string {
+		_, reqPort, _ := net.SplitHostPort(req.Host)
+		return reqPort
+	},
+	VarRequestAddr:        func(req *http.Request) string { return req.Host },
+	VarRequestPath:        func(req *http.Request) string { return req.URL.Path },
+	VarRequestQuery:       func(req *http.Request) string { return req.URL.RawQuery },
+	VarRequestURL:         func(req *http.Request) string { return req.URL.String() },
+	VarRequestURI:         func(req *http.Request) string { return req.URL.RequestURI() },
+	VarRequestContentType: func(req *http.Request) string { return req.Header.Get("Content-Type") },
+	VarRequestContentLen:  func(req *http.Request) string { return strconv.FormatInt(req.ContentLength, 10) },
+	VarRemoteHost: func(req *http.Request) string {
+		clientIP, _, err := net.SplitHostPort(req.RemoteAddr)
+		if err == nil {
+			return clientIP
+		}
+		return ""
+	},
+	VarRemotePort: func(req *http.Request) string {
+		_, clientPort, err := net.SplitHostPort(req.RemoteAddr)
+		if err == nil {
+			return clientPort
+		}
+		return ""
+	},
+	VarRemoteAddr:     func(req *http.Request) string { return req.RemoteAddr },
+	VarUpstreamName:   routes.TryGetUpstreamName,
+	VarUpstreamScheme: routes.TryGetUpstreamScheme,
+	VarUpstreamHost:   routes.TryGetUpstreamHost,
+	VarUpstreamPort:   routes.TryGetUpstreamPort,
+	VarUpstreamAddr:   routes.TryGetUpstreamAddr,
+	VarUpstreamURL:    routes.TryGetUpstreamURL,
+}
+
+var staticRespVarSubsMap = map[string]respVarGetter{
+	VarRespContentType: func(resp *ResponseModifier) string { return resp.Header().Get("Content-Type") },
+	VarRespContentLen:  func(resp *ResponseModifier) string { return strconv.Itoa(resp.ContentLength()) },
+	VarRespStatusCode:  func(resp *ResponseModifier) string { return strconv.Itoa(resp.StatusCode()) },
+}