fix(api): gracefully handle credential fetch failures in rag pipeline

[linw1995]

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3. Use the correct syntax to link this PR: Fixes #<issue number>.

Summary

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

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	0.00%	43.68%	+43.68%
Strict coverage	0.00%	43.21%	+43.21%
Typed symbols	0	22,031	+22,031
Untyped symbols	0	28,715	+28,715
Modules	0	2552	+2,552

[codecov]

Codecov Report

❌ Patch coverage is 95.55556% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.89%. Comparing base (a9bcec0) to head (d6021b8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...rvices/rag_pipeline/rag_pipeline_manage_service.py	75.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #36165   +/-   ##
=======================================
  Coverage   85.89%   85.89%           
=======================================
  Files        4454     4454           
  Lines      210844   210867   +23     
  Branches    39450    39452    +2     
=======================================
+ Hits       181104   181125   +21     
- Misses      26516    26519    +3     
+ Partials     3224     3223    -1     

Flag	Coverage Δ
api	85.34% <95.55%> (+<0.01%)	⬆️

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[hjlarry]

LGTM