Remove HERMES_API_MODE env var. api_mode is now configured where the
endpoint is defined:
- model.api_mode in config.yaml (for the active model config)
- custom_providers[].api_mode (for named custom providers)
Replace _get_configured_api_mode() with _parse_api_mode() which just
validates a value against the whitelist without reading env vars.
Both paths (model config and named custom providers) now read api_mode
from their respective config entries rather than a global override.
766f4aae2b