Custom error variant for unknown severities via `None`-friendly path

[usmanimamu17-create]

Problem Statement. Today, unknown severities produce ConfigNotFound
which the host SDK surfaces as a panic via the auto-generated client.
Bots and external tooling cannot distinguish between "config not yet
initialized" and "you passed an invalid severity".

Expected Outcome. Introduce SLAError::SeverityNotInSet (distinct
code) for unrecognized severity inputs in set_config,
calculate_sla, get_config. Update get_failure_schema accordingly.

Files: lib.rs, event_state_tests.rs,
new unit tests.