Set the TCP keepalive socket options

[zaf]

Configure the TCP keepalive timeout and interval in a way that matches
our server side behaviour.

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Note

Medium Risk
Changes the HTTP connection adapter used for all HTTPS requests, which can alter socket behavior across platforms and affect connection pooling/proxy scenarios. Functional scope is small but touches low-level networking settings.

Overview
Adds TCP keepalive configuration to outbound HTTPS requests. The client now mounts a custom TCPKeepAliveHTTPAdapter that sets socket_options (including SO_KEEPALIVE plus platform-specific TCP_KEEPIDLE/TCP_KEEPINTVL) with new defaults TCP_KEEPALIVE_IDLE_TIMEOUT=300 and TCP_KEEPALIVE_INTERVAL=60.

Adds a unit test asserting the adapter’s pool manager is configured with these keepalive options while preserving urllib3’s HTTPConnection.default_socket_options.

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