fix(webapp): abort workflow stream on human input pause

[Souravrajvi0]

Summary

Fixes #39686

When a WebApp workflow pauses for human input, the initial sendWorkflowMessage POST stream stayed open while a resume SSE (/workflow/{id}/events) was opened. Both streams could deliver conflicting events — the aborted POST stream often emitted workflow_finished with Stopped status, causing the UI to show "Stopped by user" even though the worker completed successfully.

This change:

• Tracks the active workflow stream via getAbortController
• Aborts the active stream before opening the resume SSE on workflow_paused
• Ignores stale Stopped events from the initial stream while the workflow is paused

Screenshots

Before	After
UI shows "Stopped by user" after human input resume	Workflow completes and UI shows success

Checklist

• 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 make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods