fix: SSRF proxy file descriptor leak in concurrent requests

[ZuzooVn]

Summary

This PR addresses the [Errno 9] File descriptor was closed in another greenlet error that occurs when using the SSRF proxy in concurrent workflows (e.g., greenlets, threads).

Changes:

1. Dynamic Transport Initialization

   • Fresh httpx.HTTPTransport instances are now created for each request when SSRF_PROXY_HTTP_URL and SSRF_PROXY_HTTPS_URL are configured.
   • Removes the global proxy_mounts variable to prevent shared transports across requests.

2. Concurrency Safety

   • Fixes race conditions caused by reusing the same transports in asynchronous/greenlet contexts.

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods