feat(ai): add headers option to StreamOptions for custom HTTP headers

- Added headers field to base StreamOptions interface
- Updated all providers to merge options.headers with defaults
- Forward headers and onPayload through streamSimple/completeSimple
- Bedrock not supported (uses AWS SDK auth)

packages/ai/src/providers/google-gemini-cli.ts

@@ -434,6 +434,7 @@ export const streamGoogleGeminiCli: StreamFunction<"google-gemini-cli"> = (
 				Accept: "text/event-stream",
 				...headers,
 				...(isClaudeThinkingModel(model.id) ? { "anthropic-beta": CLAUDE_THINKING_BETA_HEADER } : {}),
+				...options?.headers,
 			};
 			const requestBodyJson = JSON.stringify(requestBody);