Replace the overly-broad UPSTREAM_CONNECTION_RE regex (which matched all
provider IDs with UUID suffixes) with an x-9r-internal-models-fetch
header to detect cross-instance recursive /models fetches.
fetchCompatibleModelIds now sends the header when fetching upstream
/models; the GET handler detects it and skips dynamic fetching, breaking
the recursion loop while letting compatible providers (MLX, Ollama, vLLM)
list their models. Fixes#2626.
