Add functional tests for the loki + splunk alert plugins and the ssh deploy plugin

[LiorFink00]

Found during a full feature-coverage audit on main (fc58e08). These features work (verified live where reachable) — the gap is missing automated tests, not broken behavior.

Three plugins are fully implemented (real httpx/SSH send logic + SSRF guard) and load correctly via the loader, but no test imports/exercises them — unlike webhook/mdm which are well-tested:

• plugins/alert/loki/plugin.py — alert() builds a Loki stream payload, applies SSRF guard, POSTs. Test with a mocked httpx (assert payload shape, labels parsing, api_key header, raise-on-non-2xx, SSRF rejection).
• plugins/alert/splunk/plugin.py — alert() POSTs to HEC with token. Test payload + auth header + error handling (mirror test_webhook_plugin.py).
• plugins/deploy/ssh/plugin.py — deploy() real logic; test with a fake SSH target / mocked transport (mirror test_mdm_plugin.py + fake_jamf.py).

Note: #40 (Splunk) and #39 (SSH) are titled "Implement …" but the plugins already exist and load — those issues may be stale; this issue is specifically about test coverage.