fix: honor blue-specific llm model configuration

[l50]

Summary

• Blue workers now prefer ARES_BLUE_LLM_MODEL, then fall back to ARES_LLM_MODEL, mirroring the orchestrator's blue-only mode.
• Removed the hardcoded anthropic/claude-sonnet-4-20250514 default that caused blue workers to ask for ANTHROPIC_API_KEY when the var was unset.
• Worker errors explicitly when neither var is configured instead of silently falling back to a Claude model.

Test plan

• cargo check -p ares-cli --features blue
• Deploy to K8s and run a blue-team operation; confirm workers pick up ARES_BLUE_LLM_MODEL and no longer request ANTHROPIC_API_KEY

[l50]

Moving this to my fork — tracking in l50#28 instead.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.03%. Comparing base (f774ba1) to head (0c42609).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #354   +/-   ##
=======================================
  Coverage   80.03%   80.03%           
=======================================
  Files         433      433           
  Lines      125577   125577           
=======================================
  Hits       100500   100500           
  Misses      25077    25077           

see 1 file with indirect coverage changes

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