closes #1103

Summary (program: gssoc)

Fixes a crash when OPENAI_MODEL or ANTHROPIC_MODEL is missing from config. The change adds provider defaults directly to DEFAULT_CONFIG, so graph queries have a model to use without any extra setup.

What's New

• Default model fallbacks: Added OPENAI_MODEL (gpt-4o) and ANTHROPIC_MODEL (claude-3-5-sonnet-20241022) to DEFAULT_CONFIG in config_manager.py, so graph queries always have a model to fall back on.
  To test: Remove OPENAI_MODEL or ANTHROPIC_MODEL from ~/.codegraphcontext/.env and run a graph query. It should complete without error.
• resolve_model_name helper: Added resolve_model_name(provider, configured_value), a function that returns the configured model when set, or the provider default when the value is empty, None, or absent.
  To test: from codegraphcontext.cli.config_manager import resolve_model_name; resolve_model_name("openai", "") returns "gpt-4o"
• Tests: tests/unit/cli/test_model_defaults.py has 12 unit tests covering default values, fallback behavior, whitespace stripping, case-insensitive provider names, and unknown providers.

Feature Implementation

Testing Edge Cases 👇

Files Changed

• src/codegraphcontext/cli/config_manager.py - added OPENAI_MODEL and ANTHROPIC_MODEL to DEFAULT_CONFIG and CONFIG_DESCRIPTIONS, added resolve_model_name() helper
• tests/unit/cli/test_model_defaults.py - new file, 12 unit tests

Edge Cases Covered

• Config value is None -> falls back to provider default ✅
• Config value is "" -> falls back to provider default ✅
• Config value has surrounding whitespace (" gpt-4o ") -> stripped before returning ✅
• Provider name is mixed-case ("OpenAI", "ANTHROPIC") -> resolved case-insensitively ✅
• Unknown provider ("mistral") -> returns "", no crash ✅
• Existing non-empty config value -> returned unchanged ✅

Type of Change

• Bug fix (non-breaking)