fix: migrate model type enum construction

[WH-2099]

Summary

• replace Graphon ModelType.value_of(...) usages with direct ModelType(...) construction
• cover provider management and model load-balancing call sites
• prepare Dify for the Graphon enum alias parsing change in fix: support model type alias parsing graphon#157

Merge note

This PR is intentionally blocked until langgenius/graphon#157 is merged, released, and the Dify Graphon dependency is upgraded to that release. Before that dependency upgrade, ModelType(...) may not accept legacy provider model type aliases such as embeddings, text-generation, and reranking.

Validation

• uv run --project api ruff format --check api/core/provider_manager.py api/services/model_load_balancing_service.py api/services/model_provider_service.py
• uv run --project api ruff check api/core/provider_manager.py api/services/model_load_balancing_service.py api/services/model_provider_service.py

Closes #36549

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	44.96%	44.96%	0.00%
Strict coverage	44.48%	44.48%	0.00%
Typed symbols	23,177	23,177	0
Untyped symbols	28,682	28,682	0
Modules	2610	2610	0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.11%. Comparing base (72ee50c) to head (521545d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #36550      +/-   ##
==========================================
- Coverage   86.11%   86.11%   -0.01%     
==========================================
  Files        4483     4483              
  Lines      214917   214917              
  Branches    39897    39897              
==========================================
- Hits       185068   185066       -2     
- Misses      26457    26459       +2     
  Partials     3392     3392              

Flag	Coverage Δ
api	85.75% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.