bug: fix Duplicate HTTP headers lost in conversion

review comment - #4919 (review)

./gradlew :sentry-okhttp:test --tests="*SentryOkHttpInterceptorTest.toMap handles duplicate headers correctly*"

Author: 43jay

sentry-okhttp/src/main/java/io/sentry/okhttp/SentryOkHttpInterceptor.kt

@@ -33,6 +33,7 @@ import okhttp3.Interceptor
 import okhttp3.Request
 import okhttp3.RequestBody.Companion.toRequestBody
 import okhttp3.Response
+import org.jetbrains.annotations.VisibleForTesting
 
 /**
  * The Sentry's [SentryOkHttpInterceptor], it will automatically add a breadcrumb and start a span
@@ -263,10 +264,19 @@ public open class SentryOkHttpInterceptor(
   }
 
   /** Extracts headers from OkHttp Headers object into a map */
-  private fun okhttp3.Headers.toMap(): Map<String, String> {
+  @VisibleForTesting
+  internal fun okhttp3.Headers.toMap(): Map<String, String> {
     val headers = linkedMapOf<String, String>()
     for (i in 0 until size) {
-      headers[name(i)] = value(i)
+      val name = name(i)
+      val value = value(i)
+      val existingValue = headers[name]
+      if (existingValue != null) {
+        // Concatenate duplicate headers with comma separator
+        headers[name] = "$existingValue, $value"
+      } else {
+        headers[name] = value
+      }
     }
     return headers
   }

sentry-okhttp/src/test/java/io/sentry/okhttp/SentryOkHttpInterceptorTest.kt

@@ -680,4 +680,39 @@ class SentryOkHttpInterceptorTest {
     assertNotNull(recordedRequest.getHeader(SentryTraceHeader.SENTRY_TRACE_HEADER))
     assertNull(recordedRequest.getHeader(W3CTraceparentHeader.TRACEPARENT_HEADER))
   }
+
+  @Test
+  fun `toMap handles duplicate headers correctly`() {
+    // Create a response with duplicate headers
+    val mockResponse =
+      MockResponse()
+        .setResponseCode(200)
+        .setBody("test")
+        .addHeader("Set-Cookie", "sessionId=123")
+        .addHeader("Set-Cookie", "userId=456")
+        .addHeader("Set-Cookie", "theme=dark")
+        .addHeader("Accept", "text/html")
+        .addHeader("Accept", "application/json")
+        .addHeader("Single-Header", "value")
+
+    fixture.server.enqueue(mockResponse)
+
+    // Execute request to get response with headers
+    val sut = fixture.getSut()
+    val response = sut.newCall(getRequest()).execute()
+    val headers = response.headers
+
+    // Optional: verify OkHttp preserves duplicate headers
+    assertEquals(3, headers.values("Set-Cookie").size)
+    assertEquals(2, headers.values("Accept").size)
+    assertEquals(1, headers.values("Single-Header").size)
+
+    val interceptor = SentryOkHttpInterceptor(fixture.scopes)
+    val headerMap = with(interceptor) { headers.toMap() }
+
+    // Duplicate headers will be collapsed into 1 concatenated entry with ", " separator
+    assertEquals("sessionId=123, userId=456, theme=dark", headerMap["Set-Cookie"])
+    assertEquals("text/html, application/json", headerMap["Accept"])
+    assertEquals("value", headerMap["Single-Header"])
+  }
 }