feat(ai): add requestMetadata support to BedrockOptions for cost allocation tagging (#2511)

Add an optional requestMetadata field to BedrockOptions that forwards
key-value pairs to the Bedrock Converse API ConverseStreamCommand. Tags
appear in AWS Cost Explorer split cost allocation data, enabling callers
to attribute inference costs to specific applications or contexts.

Changes:
- Add requestMetadata?: Record<string, string> to BedrockOptions with
  JSDoc documenting AWS constraints (max 50 pairs, key 64 chars, value
  256 chars, no aws: prefix)
- Pass requestMetadata to commandInput via conditional spread to avoid
  sending undefined when omitted
- Export BedrockOptions from package root (consistent with other
  provider option types)
- Add E2E tests: metadata forwarded to SDK payload, and omitted when
  not provided

closes #2510

packages/ai/src/index.ts

@@ -4,6 +4,7 @@ export { Type } from "@sinclair/typebox";
 export * from "./api-registry.js";
 export * from "./env-api-keys.js";
 export * from "./models.js";
+export type { BedrockOptions } from "./providers/amazon-bedrock.js";
 export type { AnthropicOptions } from "./providers/anthropic.js";
 export type { AzureOpenAIResponsesOptions } from "./providers/azure-openai-responses.js";
 export type { GoogleOptions } from "./providers/google.js";