fix: var reference picker can not choose sub vars

[iamjoel]

Fix the first issue of #35730

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.66%. Comparing base (11c52e9) to head (223388d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #35732   +/-   ##
=======================================
  Coverage   85.66%   85.66%           
=======================================
  Files        4455     4455           
  Lines      208954   208957    +3     
  Branches    39036    39038    +2     
=======================================
+ Hits       178997   179005    +8     
+ Misses      26790    26785    -5     
  Partials     3167     3167           

Flag	Coverage Δ
dify-ui	93.01% <ø> (ø)
web	86.45% <100.00%> (+<0.01%)	⬆️

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