Emit retriever resources as source parts

[slegarraga]

Summary

• emit Dify message_end.metadata.retriever_resources as AI SDK document source stream parts
• preserve the original retriever resource payload in providerMetadata.difyWorkflowData.retrieverResource
• add regression coverage for streaming retriever resources

Fixes #16.

Validation

• corepack pnpm@9.15.9 test
• corepack pnpm@9.15.9 build

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Summary by cubic

Emit Dify retriever resources as document source stream parts so clients can display and attribute sources during streaming. Preserve the raw resource in providerMetadata.difyWorkflowData.retrieverResource (fixes #16).

• New Features
  • Emit message_end.metadata.retriever_resources as source parts (document, text/plain).
  • Include conversationId, messageId, and taskId in providerMetadata.difyWorkflowData.

^{Written for commit 0b0bdb9. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}