Two related fixes for custom_providers model switching:
1. validate_requested_model() now recognizes custom:<name> slugs
(e.g. custom:volcengine) as custom endpoints, not generic providers.
Previously only the bare 'custom' slug matched the relaxed validation
branch, causing model validation to fail with 'not found in provider
listing' for all named custom providers.
2. switch_model() now consults the custom_providers list when deciding
whether to override a validation rejection. If the requested model
matches the entry's 'model' field or any key in its 'models' dict,
the switch is accepted even when the remote /v1/models endpoint does
not list it.
Both changes are covered by existing tests (86 passed).
c857592558