KG-504 Change flushTextBuilder to flush whenever textBuilder is not empty (#1123)

Related to: [KG-504](https://youtrack.jetbrains.com/issue/KG-504)

## Motivation and Context
The `ContentPartsBuilder` would fail to create a `ContentPart` for text
that ended with `\n` or was followed by a DSL call like `br()` or
`newline()`. This was because the internal `flushTextBuilder` method
checked `caret.offset != 0` to detect pending text. A trailing newline
can reset the offset to 0, causing the check to fail even when the
`textBuilder` is not empty.

This change proposes to check `textBuilder.isNotEmpty()` instead,
ensuring all content is properly flushed.

## Breaking Changes
<!-- Will users need to update their code or configurations? -->
None. This is a non-breaking bug fix.

Co-authored-by: Malte Heuser <malte.heuser@ing.com>
Co-authored-by: Konstantin Pavlov <1517853+kpavlov@users.noreply.github.com>

Author: 3 people

prompt/prompt-model/src/commonMain/kotlin/ai/koog/prompt/dsl/ContentPartsBuilder.kt

@@ -70,7 +70,7 @@ public class ContentPartsBuilder : TextContentBuilderBase<List<ContentPart>>() {
      * Flushes the text builder and adds its content as a text part if there is any.
      */
     private fun flushTextBuilder() {
-        if (caret.offset != 0) {
+        if (textBuilder.isNotEmpty()) {
             parts.add(ContentPart.Text(textBuilder.toString()))
             textBuilder.clear()
         }

prompt/prompt-model/src/commonTest/kotlin/ai/koog/prompt/dsl/PromptBuilderTest.kt

@@ -5,6 +5,7 @@ import ai.koog.prompt.message.AttachmentContent
 import ai.koog.prompt.message.ContentPart
 import ai.koog.prompt.message.Message
 import ai.koog.prompt.text.text
+import io.kotest.matchers.equals.shouldBeEqual
 import kotlin.test.Ignore
 import kotlin.test.Test
 import kotlin.test.assertEquals
@@ -918,4 +919,15 @@ class PromptBuilderTest {
         assertEquals(1, prompt.messages[1].parts.size, "Should have only text part")
         assertEquals(expectedText, prompt.messages[1].parts[0], "Should have same text")
     }
+
+    @Test
+    fun testUserMessageWithTrailingNewline() {
+        val prompt = Prompt.build("test") {
+            user {
+                +"Text\n"
+            }
+        }
+
+        prompt.messages[0].parts shouldBeEqual listOf(ContentPart.Text("Text\n"))
+    }
 }