feat: add before invocation and before model calls cancellation (#785)

Author: lizradway

src/content/docs/user-guide/concepts/agents/hooks.mdx

@@ -357,6 +357,12 @@ Most event properties are read-only to prevent unintended modifications. However
 </Tab>
 <Tab label="TypeScript">
 
+- `BeforeInvocationEvent`
+    - `cancel` - Cancel the agent invocation with a message.
+
+- `BeforeModelCallEvent`
+    - `cancel` - Cancel the model call with a message.
+
 - `BeforeToolsEvent`
     - `cancel` - Cancel all tool calls in a batch with a message. See [Limit Tool Counts](#limit-tool-counts).
 

src/content/docs/user-guide/concepts/streaming/index.mdx

@@ -36,10 +36,12 @@ All streaming methods yield the same set of events:
 Each event emitted from the TypeScript agent is a class with a `type` attribute that has a unique value. When determining an event, you can use `instanceof` on the class, or an equality check on the `event.type` value. All events extend `HookableEvent`, making them both streamable and subscribable via hook callbacks.
 
 - **`BeforeInvocationEvent`**: Start of agent loop (before any iterations)
+    - **`cancel`**: Set by hook callbacks to cancel the invocation (`boolean | string`)
 - **`AfterInvocationEvent`**: End of agent loop (after all iterations complete)
     - **`error?`**: Optional error if loop terminated due to exception
 - **`BeforeModelCallEvent`**: Before model invocation
     - **`messages`**: Array of messages being sent to model
+    - **`cancel`**: Set by hook callbacks to cancel the model call (`boolean | string`)
 - **`AfterModelCallEvent`**: After model invocation
     - **`message`**: Assistant message returned by model
     - **`stopReason`**: Why generation stopped