feat(ai): add generic metadata field to StreamOptions, closes #1384

Add metadata?: Record<string, unknown> to StreamOptions so providers
can extract fields they understand. Anthropic provider extracts user_id
for abuse tracking and rate limiting. Other providers ignore it.

Based on #1384 by @7Sageer, reworked to use a generic type instead of
Anthropic-specific typing on the base interface.

packages/ai/src/providers/anthropic.ts

@@ -594,6 +594,13 @@ function buildParams(
 		}
 	}
 
+	if (options?.metadata) {
+		const userId = options.metadata.user_id;
+		if (typeof userId === "string") {
+			params.metadata = { user_id: userId };
+		}
+	}
+
 	if (options?.toolChoice) {
 		if (typeof options.toolChoice === "string") {
 			params.tool_choice = { type: options.toolChoice };