fix: preserve literal dollar signs in config parsing

[Kevin-Li-2025]

Summary

• preserve literal $ characters in config values such as regex anchors while still expanding valid environment variables
• keep the existing missing-env-var failure behavior for valid $VAR and ${VAR} placeholders
• add a regression test for a YAML regex pattern ending in $

Fixes #2349.

Root cause

string.Template.substitute() treats every $ as the start of a template placeholder and raises ValueError for a bare regex anchor like .*\.md$. The loader only needs strict validation for valid env-var placeholders; other $ characters should remain literal config content.

Tests

• uv run --package graphrag-common pytest tests/unit/load_config/test_load_config.py -q
• uv run ruff check packages/graphrag-common/graphrag_common/config/load_config.py tests/unit/load_config/test_load_config.py
• uv run ruff format --check packages/graphrag-common/graphrag_common/config/load_config.py tests/unit/load_config/test_load_config.py
• git diff --check

[Kevin-Li-2025]

Thanks for taking a look. I noticed after opening this that #2363 also addresses #2349 and keeps missing environment variable references as explicit errors while preserving literal $ characters.

This PR is a smaller current-base variant of the same fix with a focused regression test. I am happy to close this in favor of #2363 if maintainers prefer that existing PR; otherwise this is ready as an alternative patch.