fix(github): validate conflicting commit IDs and extract test helper

Address review findings from sonnet-review-bot (review 3086):

- PostReview now returns ErrConflictingCommitIDs when comments specify
  different non-empty CommitIDs, since the GitHub API accepts only a
  single commit_id per review. Previously the discrepancy was silently
  ignored, using only the first commit's ID.

- Extract newTestClient into helpers_test.go to make cross-file sharing
  between review_test.go and identity_test.go explicit.

Refs: #81

github/helpers_test.go

@@ -0,0 +1,23 @@
+package github
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+// newTestClient creates a *Client backed by an httptest.Server running the
+// given handler. The server is automatically closed when the test finishes.
+// Shared across test files in package github.
+func newTestClient(t *testing.T, handler http.HandlerFunc) *Client {
+	t.Helper()
+	srv := httptest.NewServer(handler)
+	t.Cleanup(srv.Close)
+	c := NewClient("test-token", srv.URL, AllowInsecureHTTP())
+	c.SetHTTPClient(srv.Client())
+	if err := c.SetRetryBackoff([]time.Duration{1 * time.Millisecond, 1 * time.Millisecond}); err != nil {
+		t.Fatalf("SetRetryBackoff: %v", err)
+	}
+	return c
+}