Same-provider /model switches on a 'custom' endpoint kept stale credentials
because (a) _resolve_named_custom_runtime's bare-custom + explicit_base_url
path went straight to OPENAI_API_KEY/OPENROUTER_API_KEY env fallbacks
without consulting the credential pool, and (b) switch_model() guarded
against custom-provider re-resolution to preserve base_url, locking in
the prior api_key.
Now the bare-custom path queries the credential pool first (mirroring
the named-custom-provider branch behavior), and the same-provider switch
guard is removed since resolve_runtime_provider has since grown a robust
custom-resolution path that preserves base_url from model_cfg.
Refs #18681 (the gateway-side api_key wiring is still separate),
#16254, #12919.
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