refactor(rules): change default rule from baseline to fallback behavior

The default rule should runs only when no non-default pre rule matches, instead of running first as a baseline.
This follows the old behavior as before the pr is established.:

- Default rules act as fallback handlers that execute only when no matching non-default rule exists in the pre phase
- IfElseBlockCommand now returns early when a condition matches with a nil Do block, instead of falling through to else blocks
- Add nil check for auth handler to allow requests when no auth is configured
- Fix unterminated environment variable parsing to preserve input

Updates tests to verify the new fallback behavior where special rules suppress default rule execution.

internal/route/rules/block_parser.go

@@ -77,6 +77,9 @@ func expandEnvVarsRaw(v string) (string, gperr.Error) {
 
 	var err gperr.Error
 	if inEnvVar {
+		// Write back the unterminated ${...} so the output matches the input.
+		buf.WriteString("${")
+		buf.WriteString(envVar.String())
 		err = ErrUnterminatedEnvVar
 	}
 	if len(missingEnvVars) > 0 {