Phase 2c review flagged that only 2 of the 4 structurally-identical
resolve_provider_client routing dead-ends were demoted. Complete the bug-class:
also demote+dedup the external-process ('not directly supported') and OAuth
('not directly supported, try auto') fall-throughs, keyed by provider name, so
none of the four dead-ends spam WARNING on a retry loop.
Add direct tests for the unhandled-auth_type and OAuth dedup paths via a
monkeypatched PROVIDER_REGISTRY (the review noted these were unverified).
Mutation-checked: reverting either sibling demotion fails its test.
The two fall-through branches in resolve_provider_client (unknown provider,
unhandled auth_type) logged at WARNING on every retry of a misconfigured
provider, spamming logs during retry loops. Demote both to logger.debug with
per-process dedup: the first occurrence still surfaces (a provider-name typo or
PROVIDER_REGISTRY/auth_type-drift bug is worth seeing once), while identical
repeats are suppressed for the process lifetime.
Salvaged from #56283 (extracting only the stated auxiliary_client fix; the
original PR also bundled ~2800 lines of unrelated changes across 10 other
files, which are dropped).
