Salvaged from #73811 per the consolidation triage on #76503, adapted to this
PR's per-active-provider model.reasoning_echo design.
Existing reasoning_echo tests hand-set _reasoning_echo_flag; none drives the
real config path init_agent uses:
load_config_readonly().get("model").get("reasoning_echo")
which is wrapped in `except Exception: False`, so a broken read would silently
disable the feature untested. This adds a temp-HERMES_HOME test that resolves a
named custom provider via the real resolve_runtime_provider (asserting
provider == "custom"), materializes the flag from a real config file, and
checks reasoning_content is preserved with the flag on and stripped with it off.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ab7R4NizLNbNyQZShtciKg
Caleb DeLeeuw