* test(caching): audit #44542 response cache model group key
* fix(caching): key the response cache by the router's metadata bucket only
* fix(caching): read the router's model group before the caller's metadata
* fix(caching): read caching groups only beside the router's model group
A caller's own metadata.caching_groups on /v1/responses and /v1/messages could
still pick the cache key, so a string group let one model group fill another's
entry. The key now reads caching_groups only from the metadata that holds the
router's model_group.
* test(caching): type the planted caching groups parameter
* fix(caching): read the model group from the metadata that carries the call's deployment
A router call can carry a litellm_metadata it did not write, from the auto router's classifier embedding or an SDK caller, while the router writes its own group into metadata. The cache now picks the metadata source whose model_info matches the call's deployment, then falls back to the previous order.
* fix(caching): keep main's model group lookup for calls outside the router
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Co-authored-by: mateo-berri <277851410+mateo-berri@users.noreply.github.com>
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