Mark prompt builders with PromptDSL (#200)

Currently, it is possible to invoke the role-defining functions while already being inside them, for instance:
```
user {
  system {

  }
}
```
which, despite being valid code, is confusing.

We mark the TextContentBuilder with the PromptDSL annotation so that such calls, if intended, will need to explicitly specify the receiver.

Author: adam-malek-JB

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

@@ -189,6 +189,7 @@ public class PromptBuilder internal constructor(
      *
      * This class provides methods for adding tool calls and tool results.
      */
+    @PromptDSL
     public inner class ToolMessageBuilder(public val clock: Clock) {
         /**
          * Adds a tool call message to the prompt.
@@ -199,7 +200,7 @@ public class PromptBuilder internal constructor(
          */
         @Deprecated("Use call(id, tool, content) instead", ReplaceWith("call(id, tool, content)"))
         public fun call(call: Message.Tool.Call) {
-            messages.add(call)
+            this@PromptBuilder.messages.add(call)
         }
 
         /**
@@ -225,10 +226,10 @@ public class PromptBuilder internal constructor(
          */
         @Deprecated("Use result(id, tool, content) instead", ReplaceWith("result(id, tool, content)"))
         public fun result(result: Message.Tool.Result) {
-            messages
+            this@PromptBuilder.messages
                 .indexOfLast { it is Message.Tool.Call && it.id == result.id }
                 .takeIf { it != -1 }
-                ?.let { index -> messages.add(index + 1, result) }
+                ?.let { index -> this@PromptBuilder.messages.add(index + 1, result) }
                 ?: throw IllegalStateException("Failed to add tool result: no call message with id ${result.id}")
         }
 

prompt/prompt-model/src/commonMain/kotlin/ai/koog/prompt/text/TextContentBuilder.kt

@@ -1,12 +1,15 @@
 package ai.koog.prompt.text
 
+import ai.koog.prompt.dsl.PromptDSL
+
 /**
  * A utility class for building and manipulating textual content.
  *
  * Provides methods for constructing formatted strings with features such as inserting text,
  * adding new lines, and applying padding. The builder pattern supports a fluent and convenient
  * approach to managing text content.
  */
+@PromptDSL
 public open class TextContentBuilder {
     /**
      * Represents the position of a caret within a text document.