fix(workflow): correct env variable picker validation

[lyzno1]

Summary

• fix the workflow env variable picker so existing env references are validated against the available variable pool instead of source node metadata
• keep invalid-state rendering aligned with product semantics by only showing the error icon when a selector is actually invalid
• add regression coverage for valid env references and invalid selectors in the picker helper and branch tests

Root Cause

The picker refactor in #34013 split validation and rendering logic. env selectors were still treated as valid in helper logic, but the trigger rendered the red error icon whenever outputVarNode had no title. Because environment variables are workflow-level data rather than graph nodes, they do not have source node metadata and were incorrectly flagged as errors.

Impact

• existing workflow environment variables no longer show a false invalid indicator in the variable picker
• missing env selectors still surface the invalid state once workflow data is loaded
• picker behavior now matches the prompt editor's stricter env validation semantics

Validation

• pnpm --dir web test app/components/workflow/nodes/_base/components/variable/__tests__/var-reference-picker.helpers.spec.ts app/components/workflow/nodes/_base/components/variable/__tests__/var-reference-picker.branches.spec.tsx
• pre-commit hooks: eslint, pnpm --dir web type-check:tsgo, pnpm --dir web knip

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.75%. Comparing base (75ed38f) to head (5b072b1).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #34666   +/-   ##
=======================================
  Coverage   83.75%   83.75%           
=======================================
  Files        4170     4170           
  Lines      176792   176801    +9     
  Branches    35358    35368   +10     
=======================================
+ Hits       148074   148082    +8     
- Misses      25838    25839    +1     
  Partials     2880     2880           

Flag	Coverage Δ
web	85.45% <100.00%> (+<0.01%)	⬆️

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