bridge/opencensus: add test for OTelSpanContextToOC function (#6813)

## Description
   
Adds tests for the `OTelSpanContextToOC` func in the `opencensus bridge`
as requested in issue #6561.
   
   ## Changes
   
   - Added `TestOTelSpanContextToOC` function with four test cases:
     - Empty span context conversion
     - Sampled span context conversion  
     - Not sampled span context conversion
     - Span context with tracestate conversion
   
   ## Testing
   
   The test verifies that the following fields are correctly converted:
   - TraceID 
   - SpanID
   - TraceOptions (sampled/not sampled)
   - Tracestate (when present)
   
   All existing tests continue to pass.
   
   ## Fixes
   
   Closes #6561

---------

Co-authored-by: Damien Mathieu <42@dmathieu.com>
Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

bridge/opencensus/trace_test.go

@@ -5,6 +5,7 @@ package opencensus // import "go.opentelemetry.io/otel/bridge/opencensus"
 
 import (
 	"context"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -44,3 +45,102 @@ func TestOCSpanContextToOTel(t *testing.T) {
 	got := OCSpanContextToOTel(input)
 	assert.Equal(t, want, got)
 }
+
+func TestOTelSpanContextToOC(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    oteltrace.SpanContext
+		expected octrace.SpanContext
+	}{
+		{
+			name:  "empty span context",
+			input: oteltrace.SpanContext{},
+			expected: octrace.SpanContext{
+				TraceID:      [16]byte{},
+				SpanID:       [8]byte{},
+				TraceOptions: octrace.TraceOptions(0),
+			},
+		},
+		{
+			name: "sampled span context",
+			input: oteltrace.NewSpanContext(oteltrace.SpanContextConfig{
+				TraceID:    oteltrace.TraceID{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16},
+				SpanID:     oteltrace.SpanID{1, 2, 3, 4, 5, 6, 7, 8},
+				TraceFlags: oteltrace.FlagsSampled,
+			}),
+			expected: octrace.SpanContext{
+				TraceID:      [16]byte{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16},
+				SpanID:       [8]byte{1, 2, 3, 4, 5, 6, 7, 8},
+				TraceOptions: octrace.TraceOptions(1),
+			},
+		},
+		{
+			name: "not sampled span context",
+			input: oteltrace.NewSpanContext(oteltrace.SpanContextConfig{
+				TraceID: oteltrace.TraceID{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16},
+				SpanID:  oteltrace.SpanID{1, 2, 3, 4, 5, 6, 7, 8},
+			}),
+			expected: octrace.SpanContext{
+				TraceID:      [16]byte{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16},
+				SpanID:       [8]byte{1, 2, 3, 4, 5, 6, 7, 8},
+				TraceOptions: octrace.TraceOptions(0),
+			},
+		},
+		{
+			name: "span context with tracestate",
+			input: func() oteltrace.SpanContext {
+				ts := oteltrace.TraceState{}
+				ts, _ = ts.Insert("key1", "value1")
+				ts, _ = ts.Insert("key2", "value2")
+				return oteltrace.NewSpanContext(oteltrace.SpanContextConfig{
+					TraceID:    oteltrace.TraceID{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16},
+					SpanID:     oteltrace.SpanID{1, 2, 3, 4, 5, 6, 7, 8},
+					TraceState: ts,
+				})
+			}(),
+			expected: octrace.SpanContext{
+				TraceID:      [16]byte{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16},
+				SpanID:       [8]byte{1, 2, 3, 4, 5, 6, 7, 8},
+				TraceOptions: octrace.TraceOptions(0),
+				// Tracestate will be set by the conversion function
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := OTelSpanContextToOC(tt.input)
+
+			assert.Equal(t, tt.expected.TraceID, got.TraceID, "TraceID should be correctly converted")
+
+			assert.Equal(t, tt.expected.SpanID, got.SpanID, "SpanID should be correctly converted")
+
+			assert.Equal(t, tt.expected.TraceOptions, got.TraceOptions, "TraceOptions should be correctly converted")
+
+			// Verify Tracestate is populated when input has tracestate
+			if tt.input.TraceState().Len() > 0 {
+				assert.NotNil(t, got.Tracestate, "Tracestate should be populated when input has tracestate")
+				// Verify the tracestate entries are preserved
+				expectedTraceState := tt.input.TraceState().String()
+				gotEntries := got.Tracestate.Entries()
+
+				// Convert entries back to a string representation for comparison
+				var gotTraceStateEntries []string
+				for _, entry := range gotEntries {
+					gotTraceStateEntries = append(gotTraceStateEntries, entry.Key+"="+entry.Value)
+				}
+				gotTraceState := ""
+				if len(gotTraceStateEntries) > 0 {
+					gotTraceState = strings.Join(gotTraceStateEntries, ",")
+				}
+				assert.Equal(t, expectedTraceState, gotTraceState, "Tracestate should preserve entries")
+			} else {
+				// For empty tracestate cases, ensure the field is properly handled
+				if got.Tracestate != nil {
+					entries := got.Tracestate.Entries()
+					assert.Empty(t, entries, "Empty tracestate should result in empty entries")
+				}
+			}
+		})
+	}
+}