refactor(rules): improve termination detection and block parsing logic

Refactors the termination detection in the rules DSL to properly handle if-block and if-else-block commands.

Adds new functions `commandsTerminateInPre`, `commandTerminatesInPre`, and `ifElseBlockTerminatesInPre`
to recursively check if command sequences terminate in the pre-phase.

Also improves the Parse function to try block syntax first with proper error handling and fallback to YAML.

Includes test cases for dead code detection with terminating handlers in conditional blocks.

internal/route/rules/http_flow_yaml_test.go

@@ -895,7 +895,7 @@ func TestHTTPFlow_PreTermination_SkipsLaterPreCommands_ButRunsPostOnlyAndPostMat
 	upstream := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		upstreamCalled = true
 		w.WriteHeader(http.StatusOK)
-		w.Write([]byte("upstream"))
+		fmt.Fprint(w, "upstream")
 	})
 
 	var rules Rules
@@ -903,7 +903,7 @@ func TestHTTPFlow_PreTermination_SkipsLaterPreCommands_ButRunsPostOnlyAndPostMat
 - on: path /
   do: error 403 blocked
 - on: path /
-  do: set resp_header X-Late should-not-run
+  do: set resp_header X-Late should-run
 - on: status 4xx
   do: set resp_header X-Post true
 `, &rules)
@@ -918,7 +918,7 @@ func TestHTTPFlow_PreTermination_SkipsLaterPreCommands_ButRunsPostOnlyAndPostMat
 	assert.False(t, upstreamCalled)
 	assert.Equal(t, 403, w.Code)
 	assert.Equal(t, "blocked\n", w.Body.String())
-	assert.Equal(t, "should-not-run", w.Header().Get("X-Late"))
+	assert.Equal(t, "should-run", w.Header().Get("X-Late"))
 	assert.Equal(t, "true", w.Header().Get("X-Post"))
 }