A stalled or half-open connection (e.g. a flaky local proxy that keeps
a socket ESTABLISHED but never sends data) could hang indexing forever
with no error — litellm/httpx had no timeout applied. Add a default
per-request timeout (120s, tunable via PAGEINDEX_LLM_TIMEOUT or
set_llm_params(timeout=...)) so a hung call fails fast with a clear
litellm Timeout, which the existing retry loops surface. Rides
get_llm_params(), so it flows through both llm_completion and
llm_acompletion automatically and is overridable per-index via
llm_params_scope / IndexConfig(llm_params=...).
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