Fix filter_ai_spans bypass for root spans

Remove the unconditional `return true if is_root` check in
SpanProcessor#should_forward_span? so that filters (including
filter_ai_spans) are applied to root spans as well. This aligns the
Ruby SDK behavior with the Python and JS SDKs.

Fixes #123

Author: chidiwilliams

lib/braintrust/trace/span_processor.rb

@@ -80,11 +80,6 @@ def get_parent_from_context(parent_context)
       # Determine if a span should be forwarded to the wrapped processor
       # based on configured filters
       def should_forward_span?(span)
-        # Always keep root spans (spans with no parent)
-        # Check if parent_span_id is the invalid/zero span ID
-        is_root = span.parent_span_id == OpenTelemetry::Trace::INVALID_SPAN_ID
-        return true if is_root
-
         # If no filters, keep everything
         return true if @filters.empty?
 

test/braintrust/trace/span_filter_test.rb

@@ -106,13 +106,23 @@ def test_init_with_filter_ai_spans_ignores_system_attributes
     assert_equal "gen_ai.root", spans[0].name
   end
 
-  def test_init_with_filter_ai_spans_always_keeps_root_spans
+  def test_init_with_filter_ai_spans_filters_root_spans
     exporter, tracer = setup_with_filter(filter_ai_spans: true)
 
     tracer.in_span("database.query") {}
 
     spans = exporter.finished_spans
-    assert_equal 1, spans.length, "Root spans should always be kept"
+    assert_equal 0, spans.length, "Non-AI root spans should be filtered"
+  end
+
+  def test_init_with_filter_ai_spans_keeps_ai_root_spans
+    exporter, tracer = setup_with_filter(filter_ai_spans: true)
+
+    tracer.in_span("gen_ai.completion") {}
+
+    spans = exporter.finished_spans
+    assert_equal 1, spans.length, "AI root spans should be kept"
+    assert_equal "gen_ai.completion", spans[0].name
   end
 
   def test_init_with_filter_ai_spans_drops_non_root_non_ai_spans