fix(config): preserve literal $ in config values

[jluocsa]

Description

Fixes #2349.

_parse_env_variables in graphrag-common calls
string.Template(text).substitute(os.environ). substitute() treats
every $ character as a placeholder prefix and raises
ValueError: Invalid placeholder in string for any $ that is not
followed by a valid Python identifier or {ident} / $$ escape. Common
config values fall into that trap, most notably regex anchors like
file_pattern: ".*\\.md$".

Reproduction

# settings.yaml
input:
  type: markitdown
  file_pattern: ".*\\.md$"

$ graphrag index --root <dir>
ValueError: Invalid placeholder in string: line 50, col 25

Fix

Swap substitute() for safe_substitute(). safe_substitute() leaves
unrecognized $ sequences as literal text, which is the expected
behaviour for arbitrary string config values.

To keep the existing "missing env var raises ConfigParsingError"
contract (and the test that exercises it), the function now also
inspects Template.get_identifiers() (Python 3.11+, which is graphrag's
minimum) and raises ConfigParsingError up-front if any referenced
identifier has no corresponding entry in os.environ.

Net result:

Input	Before	After
file_pattern: ".*\\.md$"	ValueError crash	preserved literally
val: $$literal	$literal	$literal (unchanged)
val: $MY_VAR (set)	substituted	substituted
val: ${MISSING}	ConfigParsingError	ConfigParsingError (unchanged)

Tests

• New unit test test_load_config_preserves_literal_dollar_sign
  covering bare $ in a regex-like value, $$ escape, and a mixed
  payload that combines a regex $ with a real env-var reference.
• Existing test_load_config_validation (which expects
  ConfigParsingError for a missing env var) still passes — the
  get_identifiers() pre-check keeps that path intact.

Local run:

.venv\Scripts\python.exe -m pytest tests/unit/load_config -q
... 3 passed in 1.98s

ruff check and ruff format --check are clean on the changed files.

Notes

• Changelog: added .semversioner/next-release/patch-…json per
  CONTRIBUTING.md.
• Diff: +1 source file, +1 test, +1 semversioner entry. No behavioural
  change for any code path that was previously succeeding.