feat: align Go/Node DSML tool-call parsing drift tolerance and update API docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/deepseek/client/client_completion_test.go

@@ -0,0 +1,36 @@
+package client
+
+import (
+	"context"
+	"errors"
+	"net/http"
+	"testing"
+
+	"ds2api/internal/auth"
+)
+
+func TestCallCompletionDoesNotFallbackForNonIdempotentCompletion(t *testing.T) {
+	var fallbackCalled bool
+	client := &Client{
+		stream: doerFunc(func(*http.Request) (*http.Response, error) {
+			return nil, errors.New("ambiguous completion write failure")
+		}),
+		fallbackS: &http.Client{Transport: roundTripperFunc(func(*http.Request) (*http.Response, error) {
+			fallbackCalled = true
+			return &http.Response{StatusCode: http.StatusOK}, nil
+		})},
+	}
+	_, err := client.CallCompletion(
+		context.Background(),
+		&auth.RequestAuth{DeepSeekToken: "token"},
+		map[string]any{"prompt": "hello"},
+		"pow",
+		3,
+	)
+	if err == nil {
+		t.Fatal("expected completion error")
+	}
+	if fallbackCalled {
+		t.Fatal("completion fallback should not be called for a non-idempotent request")
+	}
+}