[TENT] Make RPC server thread count configurable via MC_TENT_RPC_THREADS (#3535)

* [TENT] Make RPC server thread count configurable via MC_TENT_RPC_THREADS

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* [TENT] Route RPC server threads through the tent config system

MC_TENT_RPC_THREADS now maps to the rpc_server_threads config key via
ConfigHelper::loadFromEnv, so the knob works from MC_TENT_CONF too, with
validation and a [1, 1024] bound in TransferEngineImpl. The value is
passed down to CoroRpcAgent::start() instead of read via getenv.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>