fix(anthropic): handle vertex_event SSE events from Vertex AI Anthropic provider

[BillionClaw]

Problem

When using streamText with the Vertex AI Anthropic provider, streaming fails immediately because Vertex sends additional vertex_event SSE events that aren't recognized by the SDK.

These events contain usage information that cause type validation errors, resulting in the stream finishing immediately instead of properly streaming.

Solution

1. Added vertex_event to the anthropicMessagesChunkSchema with the usage field structure
2. Handle vertex_event in the transform stream (ignored like ping events)

This follows the same pattern used by other providers that handle provider-specific SSE events.

Changes

• packages/anthropic/src/anthropic-messages-api.ts: Added vertex_event schema definition
• packages/anthropic/src/anthropic-messages-language-model.ts: Added handler for vertex_event type

Fixes #13350

[vercel]

Suggested change

	z.object({
	type: z.literal('vertex_event'),
	usage: z.looseObject({
	input_tokens: z.number(),
	output_tokens: z.number(),
	cache_creation_input_tokens: z.number().nullish(),
	cache_read_input_tokens: z.number().nullish(),
	}),
	z.looseObject({
	type: z.literal('vertex_event'),

The vertex_event schema requires usage with mandatory input_tokens and output_tokens fields, but the event data is never used — causing unnecessary validation failures if Vertex AI sends vertex_event events with a different shape.

[Mati0kez]

Friendly bump -- this PR has been open for a while. Happy to make any changes if needed, or close it if no longer relevant.