Named custom providers (e.g. crof.ai) resolve to provider='custom' at the
runtime level, causing subagents to lose their intended provider identity.
On retry/fallback, resolve_provider_client('custom', model=...) searches all
providers advertising that model and picks non-deterministically, routing to
Z.AI or Bailian instead of the configured target.
The fix preserves configured_provider when runtime['provider'] == 'custom',
restoring the original provider name so routing stays correct through retries.
Adds a named constant _RUNTIME_PROVIDER_CUSTOM instead of a magic string.
Adds three regression tests:
- test_named_custom_provider_preserves_provider_name: the #26954 case
- test_standard_provider_not_overwritten_by_configured_name: openrouter/nous
must still return their own identity, not the configured name
- test_custom_provider_with_empty_configured_provider_falls_back_to_runtime:
empty provider triggers the early-return None path as before
Mind-Dragon
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