Fix User-Agent header to follow codebase conventions

Replace custom fallback "sentry-android-distribution" with error throw
when sentryClientName is null, following the pattern used throughout
the codebase where sentryClientName is expected to always be set.

Addresses PR review feedback about reusing consistent user agent.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: runningcode

sentry-android-distribution/src/main/java/io/sentry/android/distribution/DistributionHttpClient.kt

@@ -81,7 +81,7 @@ internal class DistributionHttpClient(private val options: SentryOptions) {
       connection.setRequestProperty("Accept", "application/json")
       connection.setRequestProperty(
         "User-Agent",
-        options.sentryClientName ?: "sentry-android-distribution",
+        options.sentryClientName ?: throw IllegalStateException("sentryClientName must be set"),
       )
       connection.connectTimeout = options.connectionTimeoutMillis
       connection.readTimeout = options.readTimeoutMillis