The salvaged main-agent fix (sanidhyasin) applies model.default_headers
to the primary OpenAI client, but the auxiliary client (title generation,
context compression, vision routing) builds its own clients and did not
read the override. For a `provider: custom` endpoint behind a gateway/WAF
that rejects the OpenAI SDK's identifying headers, the main turn would
succeed while auxiliary calls to the same endpoint still failed with the
opaque 502/4xx from #40033.
Add agent.auxiliary_client._apply_user_default_headers() (user values win
over provider/SDK defaults; no-op when unconfigured) and apply it at every
OpenAI-wire client construction site:
- _try_custom_endpoint() — config-level `model.provider: custom`
- the named custom-provider branch (custom_providers/providers entries),
including the anthropic-SDK-missing OpenAI-wire fallback
- the api-key-provider, async-conversion, and main resolve_provider_client
fallback branches
To prevent the two clients ever drifting on precedence/value handling,
AIAgent._apply_user_default_headers (run_agent.py) now delegates the config
read + merge to this shared helper (run_agent already imports from
auxiliary_client). Native Anthropic/Bedrock branches are untouched (they
don't use the OpenAI wire).
8 new tests (helper semantics + config-level custom + named custom);
full aux + attribution header suites green (295).
