_resolve_active_context_length() called get_model_context_length() with the
model id alone, so provider-enforced windows (e.g. Codex OAuth's 272K for
gpt-5.x vs the direct API's 1.05M) never reached the tool-search activation
gate — it sized against generic metadata for the same slug.
Resolve the runtime provider for the configured model and pass provider,
base_url, and api_key through. If credential resolution fails (offline, no
keys), degrade to a provider+base_url-only lookup so the static
provider-aware fallbacks still apply; explicit model.context_length keeps
short-circuiting as before (#46620). Gap flagged during review of #16735.
