fix(github): address review findings - remove panic, validate at config time

- MAJOR #1: Replace panic in doRequest with safe default fallback.
  Validation now happens in SetRetryBackoff (returns error on invalid
  length). doRequest gracefully falls back to default backoff if the
  configured slice is somehow invalid.

- MINOR #2: SetRetryBackoff validates slice length at configuration
  time, making the coupling between maxRetryAttempts and backoff
  explicit and catching mismatches early with a clear error.

- MINOR #4: Reword oversized response error to remove '(truncated)'
  which implied truncated data was returned when actually only an
  error is returned.

- MINOR #5: Functional options kept as-is - idiomatic Go pattern
  that allows future growth without breaking the API.

github/client_test.go

@@ -83,7 +83,9 @@ func TestDoRequest_429Retry(t *testing.T) {
 
 	c := NewClient("token", srv.URL, AllowInsecureHTTP())
 	c.SetHTTPClient(srv.Client())
-	c.SetRetryBackoff([]time.Duration{10 * time.Millisecond, 10 * time.Millisecond})
+	if err := c.SetRetryBackoff([]time.Duration{10 * time.Millisecond, 10 * time.Millisecond}); err != nil {
+		t.Fatalf("SetRetryBackoff: %v", err)
+	}
 
 	body, err := c.doGet(context.Background(), srv.URL+"/test")
 	if err != nil {
@@ -108,7 +110,9 @@ func TestDoRequest_429ExhaustsRetries(t *testing.T) {
 
 	c := NewClient("token", srv.URL, AllowInsecureHTTP())
 	c.SetHTTPClient(srv.Client())
-	c.SetRetryBackoff([]time.Duration{1 * time.Millisecond, 1 * time.Millisecond})
+	if err := c.SetRetryBackoff([]time.Duration{1 * time.Millisecond, 1 * time.Millisecond}); err != nil {
+		t.Fatalf("SetRetryBackoff: %v", err)
+	}
 
 	_, err := c.doGet(context.Background(), srv.URL+"/test")
 	if err == nil {
@@ -218,7 +222,9 @@ func TestDoRequest_429RetryAfterHeader(t *testing.T) {
 	c := NewClient("token", srv.URL, AllowInsecureHTTP())
 	c.SetHTTPClient(srv.Client())
 	// Use short backoff; Retry-After should override
-	c.SetRetryBackoff([]time.Duration{1 * time.Millisecond, 1 * time.Millisecond})
+	if err := c.SetRetryBackoff([]time.Duration{1 * time.Millisecond, 1 * time.Millisecond}); err != nil {
+		t.Fatalf("SetRetryBackoff: %v", err)
+	}
 
 	start := time.Now()
 	body, err := c.doGet(context.Background(), srv.URL+"/test")
@@ -259,7 +265,9 @@ func TestDoRequest_RetryAfterDoesNotMutateBackoff(t *testing.T) {
 
 	c := NewClient("token", srv.URL, AllowInsecureHTTP())
 	c.SetHTTPClient(srv.Client())
-	c.SetRetryBackoff([]time.Duration{1 * time.Millisecond, 1 * time.Millisecond})
+	if err := c.SetRetryBackoff([]time.Duration{1 * time.Millisecond, 1 * time.Millisecond}); err != nil {
+		t.Fatalf("SetRetryBackoff: %v", err)
+	}
 
 	_, err := c.doGet(context.Background(), srv.URL+"/test")
 	if err != nil {
@@ -297,7 +305,9 @@ func TestDoRequest_429RetryAfterHTTPDate(t *testing.T) {
 
 	c := NewClient("token", srv.URL, AllowInsecureHTTP())
 	c.SetHTTPClient(srv.Client())
-	c.SetRetryBackoff([]time.Duration{1 * time.Millisecond, 1 * time.Millisecond})
+	if err := c.SetRetryBackoff([]time.Duration{1 * time.Millisecond, 1 * time.Millisecond}); err != nil {
+		t.Fatalf("SetRetryBackoff: %v", err)
+	}
 
 	start := time.Now()
 	body, err := c.doGet(context.Background(), srv.URL+"/test")
@@ -338,7 +348,9 @@ func TestDoRequest_429RetryAfterHTTPDateInPast(t *testing.T) {
 
 	c := NewClient("token", srv.URL, AllowInsecureHTTP())
 	c.SetHTTPClient(srv.Client())
-	c.SetRetryBackoff([]time.Duration{5 * time.Second, 5 * time.Second})
+	if err := c.SetRetryBackoff([]time.Duration{5 * time.Second, 5 * time.Second}); err != nil {
+		t.Fatalf("SetRetryBackoff: %v", err)
+	}
 
 	start := time.Now()
 	_, err := c.doGet(context.Background(), srv.URL+"/test")
@@ -554,3 +566,29 @@ func TestSetHTTPClient_NilRestoresDefault(t *testing.T) {
 		t.Fatal("expected CheckRedirect policy after SetHTTPClient(nil)")
 	}
 }
+
+
+func TestSetRetryBackoff_RejectsInvalidLength(t *testing.T) {
+	c := NewClient("token", "https://api.github.com")
+
+	// Too short
+	err := c.SetRetryBackoff([]time.Duration{1 * time.Second})
+	if err == nil {
+		t.Fatal("expected error for backoff length 1")
+	}
+	if !strings.Contains(err.Error(), "backoff length 1") {
+		t.Errorf("unexpected error message: %v", err)
+	}
+
+	// Too long
+	err = c.SetRetryBackoff([]time.Duration{1 * time.Second, 2 * time.Second, 3 * time.Second})
+	if err == nil {
+		t.Fatal("expected error for backoff length 3")
+	}
+
+	// Correct length succeeds
+	err = c.SetRetryBackoff([]time.Duration{1 * time.Second, 2 * time.Second})
+	if err != nil {
+		t.Fatalf("unexpected error for valid backoff: %v", err)
+	}
+}