Add GraphRAG LLM package.

[dworthen]

Add GraphRAG LLM package.

[Copilot]

Pull request overview

This PR introduces the GraphRAG LLM package, which consolidates and refactors the language model infrastructure. The changes replace the existing graphrag.language_model module with a new graphrag-llm package, standardizing the interface for completion and embedding models across the codebase.

Changes:

• Removed the existing language model implementation and replaced it with the new graphrag-llm package
• Updated configuration models to use completion_model_id and embedding_model_id instead of generic model_id
• Refactored all workflows and operations to use the new LLM interfaces (LLMCompletion and LLMEmbedding)

Reviewed changes

Copilot reviewed 257 out of 260 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/verbs/util.py	Removed deprecated model configuration constants
tests/verbs/test_*.py	Updated test files to use new configuration utility functions
tests/unit/config/utils.py	Added new model config assertion helpers and updated default configs
tests/unit/config/test_config.py	Removed validation tests that are now handled by the LLM package
packages/graphrag/graphrag/config/models/*_config.py	Updated field names from model_id to completion_model_id/embedding_model_id
packages/graphrag/graphrag/index/workflows/*.py	Refactored to use new LLM creation functions and interfaces
packages/graphrag/graphrag/query/**/*.py	Updated query components to use new LLM interfaces
packages/graphrag/graphrag/language_model/**	Removed old language model implementation

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