Allow Redis buildsystem to recognize and link against libsystemd

[Peter-Sh]

libsystemd0 already exists in trixie base image
linking agaings libsystemd.so end enabling systemd support in Redis doesn't change its behaviour when systemd socket is not available

This allows Redis buildsystem to enable BUILD_WITH_SYSTEMD automatically by detecting according package.
Simple test is added to ensure redis is linked to libsystemd.so if it exists in the base image

For #373

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Note

Low Risk
Build-time dependency and regression-test changes only; no runtime behavior change when systemd sockets are absent.

Overview
Debian image builds now install libsystemd-dev and pkg-config during compile so Redis can auto-enable systemd integration (e.g. sd_notify) when the upstream build detects the library. The same dependency list is mirrored in debian/Dockerfile.j2.

A new entrypoint test test_redis_server_is_linked_against_libsystemd runs only when the runtime image actually ships libsystemd.so; it uses ldd on redis-server and skips on bases like Alpine that do not include systemd.

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[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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