fix(cli): ignore a half-populated socket activation environment (#11394)

A container engine started from a socket-activated system unit leaks a bare
LISTEN_PID into every container it spawns, with no matching LISTEN_FDS. LocalAI
read that as a malformed activation attempt and refused to start:

    ERROR Error running the application error=loading systemd socket
    activation listeners: invalid LISTEN_FDS ""

systemd's own sd_listen_fds() treats either variable being absent as "not
activated" rather than as an error, so do the same and fall back to ordinary
--address binding. A value that is present but malformed is still rejected, so
a real activation attempt cannot silently bind the wrong socket.

Fixes #11390


Assisted-by: Claude:claude-opus-5 [golangci-lint]

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Co-authored-by: Ettore Di Giacinto <mudler@localai.io>

docs/content/getting-started/linux.md

@@ -111,6 +111,11 @@ For a Podman-managed container, configure Podman to preserve and pass the
 systemd socket file descriptor into the container. The LocalAI process inside
 the container consumes the same activation protocol.
 
+Activation needs both `LISTEN_PID` and `LISTEN_FDS`. If only one of them is set,
+LocalAI ignores them and binds `--address` as usual. A container engine started
+from a socket-activated system unit can leak a bare `LISTEN_PID` into every
+container it spawns, and that is not an activation attempt.
+
 ## Next Steps
 
 - [Try it out with examples](/basics/try/)