feat: search — GET /search.json with grouped hits

Client.Search(term, page) url-encodes the term (operators like
@user / #category / order:latest pass through) and decodes the four
hit groups Discourse returns: posts (with blurb and highlighted
topic title), topics, users and categories. Empty term is rejected
locally with ErrInvalidRequest; server failures keep the typed
error classes. Fake-server tests cover encoding, parsing and the
403 path.

Part of Redmine 507 (Discourse Go client): search is how the 495
briefing pipeline finds landing spots and prior threads.

discourse_test.go

@@ -18,15 +18,17 @@ const testKey = "test-api-key-0123456789"
 // enforces the Api-Key/Api-Username headers, serves the endpoints the
 // client covers, and records every request for assertions.
 type fakeDiscourse struct {
-	mu       sync.Mutex
-	requests []string // "METHOD path"
-	cats     map[int]Category
-	nextCat  int
-	posts    map[int]postRec // post id -> record
-	nextPost int
-	topics   map[int]topicRec
-	nextT    int
-	srv      *httptest.Server
+	mu         sync.Mutex
+	requests   []string // "METHOD path"
+	uris       []string // "METHOD path?query" — for query assertions
+	cats       map[int]Category
+	nextCat    int
+	posts      map[int]postRec // post id -> record
+	nextPost   int
+	topics     map[int]topicRec
+	nextT      int
+	searchFail int
+	srv        *httptest.Server
 }
 
 type postRec struct {
@@ -179,6 +181,26 @@ func newFake(t *testing.T) *fakeDiscourse {
 			writeJSON(w, 405, nil)
 		}
 	})
+	mux.HandleFunc("/search.json", func(w http.ResponseWriter, r *http.Request) {
+		if !f.auth(w, r) {
+			return
+		}
+		if f.searchFail != 0 {
+			writeJSON(w, f.searchFail, map[string]any{"errors": []string{"You are not permitted to view the requested resource."}})
+			return
+		}
+		writeJSON(w, 200, map[string]any{
+			"posts": []map[string]any{{
+				"id": 55, "topic_id": 12, "username": "reachableceo", "post_number": 1,
+				"blurb":                "the fleet briefing for September",
+				"created_at":           "2026-08-29T10:00:00.000Z",
+				"topic_title_headline": "Fleet briefing 2026-09-01",
+			}},
+			"topics":     []map[string]any{{"id": 12, "title": "Fleet briefing 2026-09-01", "slug": "fleet-briefing-2026-09-01"}},
+			"users":      []map[string]any{{"id": 5, "username": "reachableceo", "name": "Charles"}},
+			"categories": []map[string]any{{"id": 2, "name": "MOPAC Briefings", "slug": "mopac-briefings"}},
+		})
+	})
 	mux.HandleFunc("/latest.json", func(w http.ResponseWriter, r *http.Request) {
 		if !f.auth(w, r) {
 			return
@@ -203,6 +225,7 @@ func newFake(t *testing.T) *fakeDiscourse {
 func (f *fakeDiscourse) auth(w http.ResponseWriter, r *http.Request) bool {
 	f.mu.Lock()
 	f.requests = append(f.requests, r.Method+" "+r.URL.Path)
+	f.uris = append(f.uris, r.Method+" "+r.URL.RequestURI())
 	f.mu.Unlock()
 	if r.Header.Get("Api-Key") != testKey || r.Header.Get("Api-Username") != "system" {
 		writeJSON(w, 403, map[string]any{"errors": []string{"Bad or missing API key"}})
@@ -222,6 +245,19 @@ func (f *fakeDiscourse) saw(substr string) bool {
 	return false
 }
 
+// sawQuery asserts a GET hit path with a query substring ("term=x",
+// "page=2").
+func (f *fakeDiscourse) sawQuery(path, param string) bool {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	for _, u := range f.uris {
+		if strings.HasPrefix(u, "GET "+path+"?") && strings.Contains(u, param) {
+			return true
+		}
+	}
+	return false
+}
+
 func writeJSON(w http.ResponseWriter, code int, body any) {
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(code)