fix: expand HTTP/Web/SDK/unit coverage

[6ba3i]

What problem does this PR solve?

This PR reduces missing coverage in the ragflow codebase by adding a small set of behavior driven tests that exercise real API and SDK contracts. It targets previously uncovered branches in HTTP API, Web API, SDK modules, and unit level auth/app helpers, focusing on request validation, error shaping, and deterministic success paths. it also adds a new testcase (test_unit) in the CI

Type of change

• Refactoring
• Other (please describe): Test coverage improvement (adds new tests and extends existing ones)

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.70%. Comparing base (88920d2) to head (0c01e2a).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
api/apps/document_app.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13110      +/-   ##
==========================================
+ Coverage   44.05%   48.70%   +4.64%     
==========================================
  Files          43       43              
  Lines        9415     9506      +91     
  Branches      107      107              
==========================================
+ Hits         4148     4630     +482     
+ Misses       5248     4843     -405     
- Partials       19       33      +14     

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