[terminal] IJPL-182482 Ignore read time during serialization

We need this `readTime` only locally on the backend and put it to the event object only for simplicity.
It is not a good design, but for now there is no suitable alternative. Ideally, we need to have the separate serializable DTO classes hierarchy for RPC.
Or switch to plain longs for measuring the time.

GitOrigin-RevId: f8e20acdfe0d856c73750711d1a14917fc9b5973

platform/execution-impl/src/com/intellij/terminal/session/TerminalOutputEvent.kt

@@ -6,6 +6,7 @@ import com.intellij.terminal.session.dto.TerminalBlocksModelStateDto
 import com.intellij.terminal.session.dto.TerminalOutputModelStateDto
 import com.intellij.terminal.session.dto.TerminalStateDto
 import kotlinx.serialization.Serializable
+import kotlinx.serialization.Transient
 import org.jetbrains.annotations.ApiStatus
 import kotlin.time.TimeMark
 
@@ -20,7 +21,9 @@ data class TerminalContentUpdatedEvent(
   val text: String,
   val styles: List<StyleRangeDto>,
   val startLineLogicalIndex: Long,
-  val readTime: TimeMark?,
+  /** This value is used only on Backend. It is always null on the Frontend. */
+  @Transient
+  val readTime: TimeMark? = null,
 ) : TerminalOutputEvent
 
 @ApiStatus.Internal