Gate review: the salvaged config_providers.py hunk was behaviour-neutral churn (the helper
already self-resolved a None route list on main) that also dropped the raw-config fallback.
Restore main's shape; since step 0c now runs for every route with a base_url, resolve via
load_config_readonly() so the per-call deepcopy of load_config() is not paid on every
context-length lookup. Comment trimmed to the WHY; the invariant test moves next to its
siblings in test_custom_provider_context_length.py.
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