feat(gitea): pass commit_id explicitly in PostReview (#107)

Add commitID parameter to gitea.Client.PostReview so the review is
anchored to the specific commit that was evaluated. The caller
(cmd/review-bot) already computes evaluatedSHA from pr.Head.Sha;
this wires it through to the Gitea API payload.

When commitID is empty, omitempty drops it from the JSON and Gitea
defaults to the current PR head (backward-compatible).

Closes #107

gitea/client_test.go

@@ -117,8 +117,9 @@ func TestPostReview(t *testing.T) {
 		}
 
 		var payload struct {
-			Body  string `json:"body"`
-			Event string `json:"event"`
+			Body     string `json:"body"`
+			Event    string `json:"event"`
+			CommitID string `json:"commit_id"`
 		}
 		if err := json.NewDecoder(r.Body).Decode(&payload); err != nil {
 			t.Fatalf("failed to decode payload: %v", err)
@@ -129,14 +130,16 @@ func TestPostReview(t *testing.T) {
 		if payload.Event != "APPROVED" {
 			t.Errorf("expected event %q, got %q", "APPROVED", payload.Event)
 		}
-
+		if payload.CommitID != "abc123def" {
+			t.Errorf("expected commit_id %q, got %q", "abc123def", payload.CommitID)
+		}
 		w.WriteHeader(http.StatusOK)
 		w.Write([]byte(`{"id":100,"user":{"login":"review-bot"},"state":"APPROVED","stale":false}`))
 	}))
 	defer server.Close()
 
 	client := NewClient(server.URL, "test-token")
-	review, err := client.PostReview(context.Background(), "owner", "repo", 3, "APPROVED", "LGTM", nil)
+	review, err := client.PostReview(context.Background(), "owner", "repo", 3, "APPROVED", "LGTM", "abc123def", nil)
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
@@ -183,12 +186,35 @@ func TestPostReview_Non200(t *testing.T) {
 	defer server.Close()
 
 	client := NewClient(server.URL, "test-token")
-	_, err := client.PostReview(context.Background(), "owner", "repo", 1, "APPROVED", "test", nil)
+	_, err := client.PostReview(context.Background(), "owner", "repo", 1, "APPROVED", "test", "", nil)
 	if err == nil {
 		t.Fatal("expected error for 403, got nil")
 	}
 }
 
+func TestPostReview_EmptyCommitID_OmittedFromPayload(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, _ := io.ReadAll(r.Body)
+		var raw map[string]interface{}
+		if err := json.Unmarshal(body, &raw); err != nil {
+			t.Fatalf("failed to decode payload: %v", err)
+		}
+		if _, exists := raw["commit_id"]; exists {
+			t.Errorf("expected commit_id to be omitted from payload when empty, but it was present")
+		}
+
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{"id":200,"user":{"login":"bot"},"state":"APPROVED","stale":false}`))
+	}))
+	defer server.Close()
+
+	client := NewClient(server.URL, "test-token")
+	_, err := client.PostReview(context.Background(), "owner", "repo", 1, "APPROVED", "ok", "", nil)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
 func TestGetFileContent(t *testing.T) {
 	expected := "# Conventions\n- Be nice\n"
 
@@ -945,8 +971,6 @@ func TestDoGet_RespectsContextCancellation(t *testing.T) {
 		t.Errorf("attempts = %d, expected 1 before context cancel during backoff", attempts)
 	}
 }
-
-
 // mockTransport is a test helper that returns errors for the first N calls,
 // then delegates to a real server.
 type mockTransport struct {