refactor!: move workflow HTTP contracts into graphon.http

[laipz8200]

Summary

• introduce a dedicated graphon.http boundary for workflow HTTP client and response contracts
• add the default HTTPX-backed HTTP client so HTTP client injection becomes optional for HTTP request, document extractor, file saver, and related LLM-style nodes
• replace workflow-facing httpx.Response leakage with graphon-owned HTTP response abstractions and reuse the shared response protocol in file runtime handling
• add focused HTTP and file-runtime tests for request normalization, retries, status handling, and default injection behavior

Breaking change:

• integrations that implement the workflow HTTP client or file runtime HTTP response contracts must now follow the graphon-owned HTTP response abstractions instead of relying on raw httpx.Response types or previous internal module boundaries

Checklist

• This pull request title follows Conventional Commits, and any breaking change is marked with !
• I ran make pre before pushing this branch
• I added or updated tests for behavior changes, or this change does not require tests
• I updated the relevant repository documentation when needed
• This change is focused and reviewable