docs: fix ReasoningMessageStartEvent role from "assistant" to "reasoning" (#1416)

The docs incorrectly documented the role field as "assistant" but both
the TypeScript and Python SDKs use "reasoning". Fixes #1329.

Author: AlemTuzlak

docs/concepts/events.mdx

@@ -610,7 +610,7 @@ displayed to users (e.g., a summary or partial chain-of-thought).
 | Property    | Description                                   |
 | ----------- | --------------------------------------------- |
 | `messageId` | Unique identifier of the message              |
-| `role`      | Role of the reasoning message (`"assistant"`) |
+| `role`      | Role of the reasoning message (`"reasoning"`) |
 
 ### ReasoningMessageContent
 

docs/concepts/reasoning.mdx

@@ -149,7 +149,7 @@ yield {
 yield {
   type: "REASONING_MESSAGE_START",
   messageId: "msg-123",
-  role: "assistant",
+  role: "reasoning",
 }
 
 yield {
@@ -205,7 +205,7 @@ yield {
 yield {
   type: "REASONING_MESSAGE_START",
   messageId: "msg-456",
-  role: "assistant",
+  role: "reasoning",
 }
 
 yield {

docs/sdk/js/core/events.mdx

@@ -429,14 +429,14 @@ the message history.
 type ReasoningMessageStartEvent = BaseEvent & {
   type: EventType.REASONING_MESSAGE_START
   messageId: string
-  role: "assistant"
+  role: "reasoning"
 }
 ```
 
-| Property    | Type          | Description                       |
-| ----------- | ------------- | --------------------------------- |
-| `messageId` | `string`      | Unique identifier for the message |
-| `role`      | `"assistant"` | Role is always "assistant"        |
+| Property    | Type          | Description                        |
+| ----------- | ------------- | ---------------------------------- |
+| `messageId` | `string`      | Unique identifier for the message  |
+| `role`      | `"reasoning"` | Role is always "reasoning"         |
 
 ### ReasoningMessageContentEvent
 

docs/sdk/python/core/events.mdx

@@ -470,13 +470,13 @@ the message history.
 class ReasoningMessageStartEvent(BaseEvent):
     type: Literal[EventType.REASONING_MESSAGE_START]
     message_id: str
-    role: Literal["assistant"]
+    role: Literal["reasoning"]
 ```
 
-| Property     | Type                   | Description                       |
-| ------------ | ---------------------- | --------------------------------- |
-| `message_id` | `str`                  | Unique identifier for the message |
-| `role`       | `Literal["assistant"]` | Role is always "assistant"        |
+| Property     | Type                    | Description                       |
+| ------------ | ----------------------- | --------------------------------- |
+| `message_id` | `str`                   | Unique identifier for the message |
+| `role`       | `Literal["reasoning"]`  | Role is always "reasoning"        |
 
 ### ReasoningMessageContentEvent