fix: honor blue-specific llm model configuration

[l50]

Key Changes:

• Prioritizes ARES_BLUE_LLM_MODEL for blue worker LLM selection
• Falls back to ARES_LLM_MODEL only when the blue-specific override is unset
• Fails fast with a clear error when no blue worker LLM model is configured

Added:

• Blue worker model override support - Added ARES_BLUE_LLM_MODEL handling in ares-cli/src/worker/mod.rs to match blue-only orchestrator configuration
• Empty value validation - Ignores empty environment variable values so misconfigured model settings do not get passed to provider creation

Changed:

• Blue worker LLM resolution - Replaced the hardcoded default model fallback with explicit environment-based configuration using ARES_BLUE_LLM_MODEL, then ARES_LLM_MODEL
• Missing model handling - Returns a descriptive error when neither supported environment variable is set, making configuration issues easier to diagnose

Removed:

• Implicit default LLM model - Removed the automatic fallback to anthropic/claude-sonnet-4-20250514 for blue worker mode