fix(config): use safe_substitute to prevent crashes on $ in config values (fixes #2349)

[hanhan761]

Summary

Replace substitute() with safe_substitute() in config env-var parsing so that bare $ characters (e.g. regex file_pattern: ".*\\.txt$") don't crash the config loader.

Changes

• In graphrag-common/graphrag_common/config/load_config.py, replaced Template(text).substitute(os.environ) with Template(text).safe_substitute(os.environ)
• Removed the now-unnecessary try/except KeyError wrapper, since safe_substitute() never raises KeyError or ValueError

Issue

Fixes #2349

Verification

>>> from string import Template
>>> t = Template('.*\\.txt$')
>>> t.safe_substitute({})
'.*\\.txt$'

safe_substitute() leaves unrecognized $ patterns as literal text instead of raising ValueError.