BetaManagedAgentsEventParamsOfUserMessage helper omits required Type field

## Bug

The helper function `BetaManagedAgentsEventParamsOfUserMessage()` in `betasessionevent.go` does not set the required `Type` field on `BetaManagedAgentsUserMessageEventParams`, resulting in a `400 Bad Request` with `events[0].type: Field required` when calling `POST /v1/sessions/{id}/events`.

## Reproduction

```go
client.Beta.Sessions.Events.Send(ctx, sessionID, anthropic.BetaSessionEventSendParams{
Events: []anthropic.BetaManagedAgentsEventParamsUnion{
anthropic.BetaManagedAgentsEventParamsOfUserMessage(
[]anthropic.BetaManagedAgentsUserMessageEventParamsContentUnion{{
OfText: &anthropic.BetaManagedAgentsTextBlockParam{
Text: "hello",
Type: anthropic.BetaManagedAgentsTextBlockTypeText,
},
}},
),
},
})
```

Returns:
```
400 Bad Request: {"type":"error","error":{"type":"invalid_request_error","message":"Failed to parse request: events[0].type: Field required"}}
```

## Root cause

The helper only sets `Content`, not `Type`:

```go
func BetaManagedAgentsEventParamsOfUserMessage(content []BetaManagedAgentsUserMessageEventParamsContentUnion) BetaManagedAgentsEventParamsUnion {
var userMessage BetaManagedAgentsUserMessageEventParams
userMessage.Content = content // ✅
// userMessage.Type is never set ❌
return BetaManagedAgentsEventParamsUnion{OfUserMessage: &userMessage}
}
```

Compare with the other helpers which DO accept and set their type parameter:

```go
func BetaManagedAgentsEventParamsOfUserInterrupt(type_ ...) { ... userInterrupt.Type = type_ ... }
func BetaManagedAgentsEventParamsOfUserToolConfirmation(result, toolUseID, type_ ...) { ... userToolConfirmation.Type = type_ ... }
```

## Suggested fix

Either set the type automatically (since there's only one valid value):

```go
func BetaManagedAgentsEventParamsOfUserMessage(content []BetaManagedAgentsUserMessageEventParamsContentUnion) BetaManagedAgentsEventParamsUnion {
var userMessage BetaManagedAgentsUserMessageEventParams
userMessage.Content = content
userMessage.Type = BetaManagedAgentsUserMessageEventParamsTypeUserMessage
return BetaManagedAgentsEventParamsUnion{OfUserMessage: &userMessage}
}
```

## Workaround

Construct the struct directly instead of using the helper:

```go
Events: []anthropic.BetaManagedAgentsEventParamsUnion{
{
OfUserMessage: &anthropic.BetaManagedAgentsUserMessageEventParams{
Content: content,
Type: anthropic.BetaManagedAgentsUserMessageEventParamsTypeUserMessage,
},
},
},
```

## Version

`anthropic-sdk-go v1.34.0`