[Moonpuck]

What this does

The socket pool already expires idle connections, but there was no way to ask
for it: the HTTP client always built its pool with the default expiration
(-1ULL, never), and objstore::ClientOptions had no knob for it either.

This exposes it end to end:

• Client::set_max_idle_timeout() / Client::max_idle_timeout(), captured by
  each vCPU's built-in dialer on its first dial
• new_http_dialer() forwards the expiration to its PoolDialer
• objstore::ClientOptions::max_idle_timeout_us, passed down to the HTTP client

Default stays -1ULL (never expire), so existing behaviour is unchanged.

Hardening: a zero expiration is now rejected

SocketPoolArgs::expiration == 0 was harmless before only because nothing ever
passed it. If it reaches the pool, collect() computes
interval = min(heartbeat_interval, 0) = 0 and check_expire_heartbeat() trips
assert(near_expire.expiration() > now) -- or spins the collector burning a
vCPU in release builds, where the assert is compiled out.

Now that the value is user-reachable, TCPSocketPool's constructor normalizes
0 to -1ULL. Clamping there rather than in new_http_dialer() covers every
caller of the public new_tcp_socket_pool() API, not just the HTTP path.

Tests

net/http/test/client_function_test.cpp:

• max_idle_timeout_expires_pooled_connection
• zero_max_idle_timeout_means_no_expiration

As a negative control, removing the clamp makes the second test abort with
Assertion failed: (near_expire.expiration() > now), function check_expire_heartbeat, file net/pooled_socket.cpp.

Local (macOS / kqueue, Debug):

• client_function_test: 55 passed, 1 skipped
• dialer_test: 17 passed
• test-sockpool: 32 passed

Note

There is a separate, hand-written PR against release/0.9 for the same feature.
It is not a cherry-pick: 0.9 has no net/http/dialer.*, and its pool is a
thread_local PooledDialer inside client.cpp, so the plumbing and the test
setup are both different. Please do not add need-backport here -- the
automatic cherry-pick would conflict on dialer.{h,cpp} (modify/delete) and
commit the conflict markers into the generated PR.