test: Add companion ui tests for useChat from upstream

[lqhuang]

As title said.

What I did:

1. Fix setData test cases
2. Fix handler callbacks which due to incorrect use of atom with functions
   • In previous implement, I forgot () => value as atom value would execute directly.
   • Now function is passed into atom with an object (atom({ fn: userFunc }))
3. Fix dependencies of a useCallback function (in handleSubmit)
4. Refactor allowEmptySubmit with non-empty input and attachments
5. Add atomWithReset
   • because state of atoms is out of control for useChat, here I add resettable atoms and <ResetMessageComponent /> (I don't know how to reset out of react context) to clean up all state between each unit test.
   • this is a non-deterministic feature, may remove in the future
   • (remove or not) It depends on how would we actually call useChat in real case.

Preview test state:

 ❯ src/react/use-chat.ui.test.tsx
   ✓ data protocol stream (9)
     ✓ should show streamed response
     × should set stream data
     ✓ setData (2)
       ✓ should set data
       × should clear data
     ✓ should show error response when there is a server error
     ✓ should show error response when there is a streaming error
     ✓ loading state (2)
       ✓ should show loading state
       ✓ should reset loading state on error
     × should invoke onFinish when the stream finishes

Current test state:

 ❯ src/react/use-chat.ui.test.tsx (28) 718ms
   ✓ data protocol stream (9)
     ✓ should show streamed response
     ✓ should set stream data
     ✓ setData (2)
       ✓ should set data
       ✓ should clear data
     ✓ should show error response when there is a server error
     ✓ should show error response when there is a streaming error
     ✓ loading state (2)
       ✓ should show loading state
       ✓ should reset loading state on error
     ✓ should invoke onFinish when the stream finishes
   ✓ text stream (3)
     ✓ should show streamed response
     ✓ should have stable message ids
     ✓ should invoke onFinish when the stream finishes
   ✓ form actions (1)
     ✓ should show streamed response using handleSubmit
   ✓ form actions (with options) (1)
     ✓ allowEmptySubmit
   ✓ prepareRequestBody (1)
     ✓ should show streamed response
   ✓ onToolCall (1)
     ✓ should invoke onToolCall when a tool call is received from the server's response
   ✓ tool invocations (3)
     ✓ should display partial tool call, tool call, and tool result
     ✓ should display partial tool call and tool result (when there is no tool call streaming)
     ✓ should update tool call to result when addToolResult is called
   ✓ maxSteps (2)
     ✓ two steps with automatic tool call (1)
       ✓ should automatically call api when tool call gets executed via onToolCall
     ✓ two steps with error response (1)
       ✓ should automatically call api when tool call gets executed via onToolCall
   ✓ file attachments with data url (2)
     ✓ should handle text file attachment and submission
     ✓ should handle image file attachment and submission
   ✓ file attachments with url (1)
     ✓ should handle image file attachment and submission
   ✓ attachments with empty submit (1)
     ✓ should handle image file attachment and submission
   ✓ should append message with attachments (1)
     ✓ should handle image file attachment and submission
   ✓ reload (1)
     ✓ should show streamed response
   ✓ test sending additional fields during message submission (1)
     ✓ annotations

[lqhuang]

Ready for review @himself65

[himself65]

Thanks for helping!