Reuse proxy backend sessions within requests - #4860

Proxy component resolution and execution previously opened separate backend client lifetimes, so one frontend call could negotiate with the backend more than once. ProxyProvider now lazily retains one isolated backend session for the root request and disconnects it during request cleanup; concurrent requests remain isolated and local component calls do not wake the backend.

The default is request scope, while stateful integrations can preserve their deliberate connection lifecycle explicitly:

provider = ProxyProvider(lambda: ProxyClient(backend))
stateful_provider = ProxyProvider(
    stateful_client.new_stateful,
    session_scope="operation",
)

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