ClassNotFoundException for UserAgent plugin with R8 enabled (Ktor 3.3.2)

## Ktor Version
3.3.2

## Ktor Engine Used
ktor-client-android

## Kotlin Version
2.2.21

## Operating System
Android 13 (also affects other upper API level Android versions)

## Describe the Bug

When a third-party SDK (Moloco SDK) initializes an HttpClient with the UserAgent plugin,
the app crashes with `ClassNotFoundException` on release builds with R8 enabled.

This issue affected 27 users over 1 day, occurring on Samsung Galaxy devices.

## Stack Trace

Fatal Exception: java.lang.NoClassDefFoundError
Failed resolution of: Lio/ktor/client/plugins/UserAgent;

com.moloco.sdk.internal.http.a$a.a (SourceFile:1)
com.moloco.sdk.internal.http.a$a.invoke (SourceFile:1)
io.ktor.client.HttpClientKt.HttpClient (HttpClient.kt:648)
io.ktor.client.HttpClientJvmKt.HttpClient (HttpClientJvm.kt:25)
...

Caused by java.lang.ClassNotFoundException
io.ktor.client.plugins.UserAgent

## Expected Behavior

The `UserAgent` class should be available at runtime and not stripped by R8.

## Steps to Reproduce

1. Add Ktor 3.3.2 dependencies to an Android project
2. Enable R8 minification (`minifyEnabled = true`)
3. Use a library (e.g., Moloco SDK) that initializes HttpClient with UserAgent plugin
4. Build and run the release APK
5. App crashes when HttpClient is initialized

## Workaround

Downgrading to Ktor 2.3.12 resolves the issue.

## Additional Context

- I saw that Ktor 3.2.2 included an "Android D8 compatibility fix"
- However, this issue persists in 3.3.2
- Our ProGuard rules already include `-keep class io.ktor.client.plugins.** { *; }`
- The issue may be related to how R8 handles Ktor's internal class references

## Environment Details

```kotlin
// gradle/libs.versions.toml
ktor = "3.3.2"

// app/build.gradle.kts
implementation("io.ktor:ktor-client-android:3.3.2")
implementation("io.ktor:ktor-client-content-negotiation:3.3.2")
implementation("io.ktor:ktor-client-logging:3.3.2")
implementation("io.ktor:ktor-client-auth:3.3.2")

Questions

1. Is this a known issue with Ktor 3.3.2?
2. Are there additional ProGuard/R8 rules needed for the UserAgent plugin?
3. Should ktor-client-core explicitly expose the UserAgent class for R8 compatibility?

Start with app/build.gradle.kts and gradle/libs.versions.toml, then reproduce the failure by building the minified Android release APK and initializing HttpClient with UserAgent. Trace the UserAgent reference from HttpClient.kt:648 and compare it with the existing R8 keep rule. Done means the release APK initializes without ClassNotFoundException.

Written by the indexing model from the issue text.