Introduce a version-agnostic credential directory (~/.unraid-mcp, overridable
via UNRAID_CREDENTIALS_DIR env var) and surface it as CREDENTIALS_DIR and
CREDENTIALS_ENV_PATH module-level constants. Prepend the canonical .env path to
dotenv_paths so all runtimes (plugin, uv, Docker) resolve credentials from the
same stable location without relying on versioned plugin cache paths.
make_graphql_request now reads credentials from the settings module at call
time (via a local import) instead of relying on module-level names captured at
import time. When either credential is missing it raises CredentialsNotConfiguredError
(not ToolError), allowing callers to trigger elicitation rather than surfacing a
generic error to the MCP client.
Updated tests/test_client.py and tests/http_layer/test_request_construction.py
to patch unraid_mcp.config.settings.* instead of the now-removed client-module
attrs, and to expect CredentialsNotConfiguredError on missing credentials.
