Code Review Bench PR #30832 - fix(api): defer streaming response until referenced variables are updated

[tomerqodo]

Code Review Bench PR langgenius#30832

Original PR Title: fix(api): defer streaming response until referenced variables are updated
Original PR Description: > [!IMPORTANT]

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Summary

Fix langgenius#30831.

Root cause: VariableAssigner v1 does not override blocks_variable_output, so the response streaming coordinator treats it as non-blocking and may start streaming Answer nodes before the conversation variable update is applied.

Screenshots

Before	After

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 and make type-check (backend) and cd web && npx lint-staged (frontend) to appease the lint gods

Original PR URL: langgenius#30832