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/CHANGELOG.md

@@ -2,6 +2,11 @@
 
 ## [Unreleased]
 
+### Added
+
+- Added `requestMetadata` option to `BedrockOptions` for AWS cost allocation tagging; key-value pairs are forwarded to the Bedrock Converse API `requestMetadata` field and appear in AWS Cost Explorer split cost allocation data.
+- Exported `BedrockOptions` type from the package root entry point, consistent with other provider option types.
+
 ### Fixed
 
 - Fixed OpenAI Responses replay for foreign tool-call item IDs by hashing foreign `function_call.id` values into bounded `fc_<hash>` IDs instead of preserving backend-specific normalized shapes that OpenAI Codex rejects.