feat: add DuckDuckGo, GitHub, Reddit, and Bing engines

- DuckDuckGo: scrapes Lite HTML endpoint for results
  - Language-aware region mapping (de→de-de, ja→jp-jp, etc.)
  - HTML parser extracts result links and snippets from DDG Lite markup
  - Shared html_helpers.go with extractAttr, stripHTML, htmlUnescape

- GitHub: uses public Search API (repos, sorted by stars)
  - No auth required (10 req/min unauthenticated)
  - Shows stars, language, topics, last updated date
  - Paginated via GitHub's page parameter

- Reddit: uses public JSON search API
  - Respects safesearch (skips over_18 posts)
  - Shows subreddit, score, comment count
  - Links self-posts to the thread URL

- Bing: scrapes web search HTML (b_algo containers)
  - Extracts titles, URLs, and snippets from Bing's result markup
  - Handles Bing's tracking URL encoding

- Updated factory, config defaults, and config.example.toml
- Full test suite: unit tests for all engines, HTML parsing tests,
  region mapping tests, live request tests (skipped in short mode)

9 engines total: wikipedia, arxiv, crossref, braveapi, qwant,
duckduckgo, github, reddit, bing

internal/engines/bing_test.go

@@ -0,0 +1,91 @@
+package engines
+
+import (
+	"context"
+	"net/http"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/ashie/gosearch/internal/contracts"
+)
+
+func TestBingEngine_EmptyQuery(t *testing.T) {
+	eng := &BingEngine{}
+	resp, err := eng.Search(context.Background(), contracts.SearchRequest{Query: ""})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(resp.Results) != 0 {
+		t.Errorf("expected 0 results for empty query, got %d", len(resp.Results))
+	}
+}
+
+func TestBingEngine_Name(t *testing.T) {
+	eng := &BingEngine{}
+	if eng.Name() != "bing" {
+		t.Errorf("expected 'bing', got %q", eng.Name())
+	}
+}
+
+func TestBingEngine_Uninitialized(t *testing.T) {
+	eng := &BingEngine{}
+	_, err := eng.Search(context.Background(), contracts.SearchRequest{Query: "test"})
+	if err == nil {
+		t.Error("expected error for uninitialized client")
+	}
+}
+
+func TestParseBingHTML(t *testing.T) {
+	html := `<li class="b_algo">
+		<h2><a href="https://example.com">Example Title</a></h2>
+		<div class="b_caption"><p>This is a test snippet from Bing.</p></div>
+	</li>
+	<li class="b_algo">
+		<h2><a href="https://example2.com">Second Result</a></h2>
+		<div class="b_caption"><p>Another snippet</p></div>
+	</li>`
+
+	results, err := parseBingHTML(strings.NewReader(html))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(results) != 2 {
+		t.Fatalf("expected 2 results, got %d", len(results))
+	}
+	if results[0].Title != "Example Title" {
+		t.Errorf("expected 'Example Title', got %q", results[0].Title)
+	}
+	if *results[0].URL != "https://example.com" {
+		t.Errorf("expected 'https://example.com', got %q", *results[0].URL)
+	}
+	if results[0].Content != "This is a test snippet from Bing." {
+		t.Errorf("unexpected content: %q", results[0].Content)
+	}
+}
+
+func TestBingEngine_LiveRequest(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping live request")
+	}
+
+	client := &http.Client{}
+	eng := &BingEngine{client: client}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
+	defer cancel()
+
+	resp, err := eng.Search(ctx, contracts.SearchRequest{
+		Query: "golang programming language",
+	})
+	if err != nil {
+		t.Fatalf("live search failed: %v", err)
+	}
+
+	t.Logf("bing returned %d results", len(resp.Results))
+	for _, r := range resp.Results {
+		if r.Engine != "bing" {
+			t.Errorf("expected engine 'bing', got %q", r.Engine)
+		}
+	}
+}