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/errors.go

@@ -5,18 +5,25 @@ import (
 )
 
 var (
-	ErrUnterminatedQuotes     = gperr.New("unterminated quotes")
-	ErrUnterminatedBrackets   = gperr.New("unterminated brackets")
-	ErrUnterminatedEnvVar     = gperr.New("unterminated env var")
-	ErrUnknownDirective       = gperr.New("unknown directive")
-	ErrUnknownModField        = gperr.New("unknown field")
-	ErrEnvVarNotFound         = gperr.New("env variable not found")
-	ErrInvalidArguments       = gperr.New("invalid arguments")
-	ErrInvalidOnTarget        = gperr.New("invalid `rule.on` target")
-	ErrInvalidCommandSequence = gperr.New("invalid command sequence")
+	ErrUnterminatedQuotes      = gperr.New("unterminated quotes")
+	ErrUnterminatedBrackets    = gperr.New("unterminated brackets")
+	ErrUnterminatedParenthesis = gperr.New("unterminated parenthesis")
+	ErrUnterminatedEnvVar      = gperr.New("unterminated env var")
+	ErrUnknownDirective        = gperr.New("unknown directive")
+	ErrUnknownModField         = gperr.New("unknown field")
+	ErrEnvVarNotFound          = gperr.New("env variable not found")
+	ErrInvalidArguments        = gperr.New("invalid arguments")
+	ErrInvalidOnTarget         = gperr.New("invalid `rule.on` target")
+	ErrInvalidCommandSequence  = gperr.New("invalid command sequence")
+
+	// vars errors
+	ErrNoArgProvided   = gperr.New("no argument provided")
+	ErrUnexpectedVar   = gperr.New("unexpected variable")
+	ErrUnexpectedQuote = gperr.New("unexpected quote")
 
 	ErrExpectNoArg          = gperr.Wrap(ErrInvalidArguments, "expect no arg")
 	ErrExpectOneArg         = gperr.Wrap(ErrInvalidArguments, "expect 1 arg")
+	ErrExpectOneOrTwoArgs   = gperr.Wrap(ErrInvalidArguments, "expect 1 or 2 args")
 	ErrExpectTwoArgs        = gperr.Wrap(ErrInvalidArguments, "expect 2 args")
 	ErrExpectTwoOrThreeArgs = gperr.Wrap(ErrInvalidArguments, "expect 2 or 3 args")
 	ErrExpectThreeArgs      = gperr.Wrap(ErrInvalidArguments, "expect 3 args")