fix: use posthog-java/server User-Agent for server-side runtime detection

Workaround: PostHog's runtime detection uses User-Agent patterns to determine
if requests come from server-side or client-side SDKs. The pattern
"posthog-server/" is not recognized, so we use "posthog-java/server/" instead
until PostHog adds support for "posthog-server/" in their detection patterns.

Also adds $lib and $lib_version to flags requests for SDK identification,
and makes userAgent configurable in PostHogConfig.

Author: Igor Macedo Quintanilha

posthog-server/src/main/java/com/posthog/server/PostHogConfig.kt

@@ -194,6 +194,9 @@ public open class PostHogConfig constructor(
         // Set SDK identification
         coreConfig.sdkName = BuildConfig.SDK_NAME
         coreConfig.sdkVersion = BuildConfig.VERSION_NAME
+        // Workaround: Use "posthog-java/server" for User-Agent for server-side runtime detection
+        // until PostHog supports "posthog-server/" in their runtime detection patterns.
+        coreConfig.userAgent = "posthog-java/server/${BuildConfig.VERSION_NAME}"
         coreConfig.context = PostHogServerContext(coreConfig)
 
         return coreConfig

posthog/src/main/java/com/posthog/PostHogConfig.kt

@@ -277,10 +277,13 @@ public open class PostHogConfig(
     @PostHogInternal
     public var sdkVersion: String = BuildConfig.VERSION_NAME
 
-    internal val userAgent: String
-        get() {
-            return "$sdkName/$sdkVersion"
-        }
+    // Can be overridden by SDK modules for custom User-Agent header
+    @PostHogInternal
+    public var userAgent: String? = null
+
+    internal fun getUserAgent(): String {
+        return userAgent ?: "$sdkName/$sdkVersion"
+    }
 
     @PostHogInternal
     public var legacyStoragePrefix: String? = null

posthog/src/main/java/com/posthog/internal/PostHogApi.kt

@@ -113,7 +113,7 @@ public class PostHogApi(
 
         return Request.Builder()
             .url(url)
-            .header("User-Agent", config.userAgent)
+            .header("User-Agent", config.getUserAgent())
             .post(requestBody)
             .build()
     }
@@ -135,6 +135,8 @@ public class PostHogApi(
                 personProperties,
                 groupProperties,
                 config.evaluationContexts,
+                lib = config.sdkName,
+                libVersion = config.sdkVersion,
             )
 
         val url = "$theHost/flags/?v=2&config=true"
@@ -178,7 +180,7 @@ public class PostHogApi(
         val request =
             Request.Builder()
                 .url("$host/array/${config.apiKey}/config")
-                .header("User-Agent", config.userAgent)
+                .header("User-Agent", config.getUserAgent())
                 .header("Content-Type", APP_JSON_UTF_8)
                 .get()
                 .build()
@@ -220,7 +222,7 @@ public class PostHogApi(
         val requestBuilder =
             Request.Builder()
                 .url(url)
-                .header("User-Agent", config.userAgent)
+                .header("User-Agent", config.getUserAgent())
                 .header("Content-Type", APP_JSON_UTF_8)
                 .header("Authorization", "Bearer $personalApiKey")
 
@@ -266,6 +268,16 @@ public class PostHogApi(
     private fun logResponse(response: Response): Response {
         if (config.debug) {
             try {
+                // Log response headers
+                val responseHeaders = response.headers
+                val responseHeaderStrings = responseHeaders.names().map { name -> "$name: ${responseHeaders[name]}" }
+                config.logger.log("Response headers for ${response.request.url}: ${responseHeaderStrings.joinToString(", ")}")
+
+                // Log the final request headers (after interceptors like gzip)
+                val finalRequestHeaders = response.request.headers
+                val finalRequestHeaderStrings = finalRequestHeaders.names().map { name -> "$name: ${finalRequestHeaders[name]}" }
+                config.logger.log("Final request headers for ${response.request.url}: ${finalRequestHeaderStrings.joinToString(", ")}")
+
                 val responseBody = response.body ?: return response
                 val mediaType = responseBody.contentType()
                 val content =

posthog/src/main/java/com/posthog/internal/PostHogFlagsRequest.kt

@@ -11,10 +11,18 @@ internal class PostHogFlagsRequest(
     personProperties: Map<String, Any?>? = null,
     groupProperties: Map<String, Map<String, Any?>>? = null,
     evaluationContexts: List<String>? = null,
+    lib: String? = null,
+    libVersion: String? = null,
 ) : HashMap<String, Any>() {
     init {
         this["api_key"] = apiKey
         this["distinct_id"] = distinctId
+        if (!lib.isNullOrBlank()) {
+            this["\$lib"] = lib
+        }
+        if (!libVersion.isNullOrBlank()) {
+            this["\$lib_version"] = libVersion
+        }
         if (!anonymousId.isNullOrBlank()) {
             this["\$anon_distinct_id"] = anonymousId
         }

posthog/src/test/java/com/posthog/PostHogConfigTest.kt

@@ -86,14 +86,20 @@ internal class PostHogConfigTest {
     }
 
     @Test
-    fun `user agent is returned correctly if changed`() {
-        config.sdkName = "posthog-android"
-        assertEquals("posthog-android/${BuildConfig.VERSION_NAME}", config.userAgent)
+    fun `user agent is returned correctly if overridden`() {
+        config.userAgent = "posthog-android/1.0.0"
+        assertEquals("posthog-android/1.0.0", config.getUserAgent())
     }
 
     @Test
-    fun `user agent is set the java sdk by default`() {
-        assertEquals("posthog-java/${BuildConfig.VERSION_NAME}", config.userAgent)
+    fun `user agent falls back to sdkName and sdkVersion by default`() {
+        assertEquals("posthog-java/${BuildConfig.VERSION_NAME}", config.getUserAgent())
+    }
+
+    @Test
+    fun `user agent falls back to sdkName when userAgent is null`() {
+        config.sdkName = "posthog-android"
+        assertEquals("posthog-android/${BuildConfig.VERSION_NAME}", config.getUserAgent())
     }
 
     @Test