fix(api): suppress stuck-instance alerts for hosts in maintenance mode

[kdhulipala-wq]

Description

Stuck Assigned-substate machines sometimes take days to resolve, and operators put them into maintenance mode to silence the on-call. The existing setup does not suppress alerts for these machines when they are put into maintenance mode, and the oncall personnel is still receiving the PD alerts.

This PR makes SetMaintenance::Enable also write ExcludeFromStateMachineSla — matching what the admin-cli InternalMaintenance template has been doing — so state_sla() short-circuits to no_sla() and a host in maintenance stops contributing to stuck-instance alerts regardless of which state or substate it's in.

This PR also adds a regression test (test_maintenance_suppresses_state_machine_sla_alert) that forces a host into the zero-SLA Failed state, confirms it reports above-SLA, enables maintenance, confirms the breach flips off, then disables maintenance and confirms it flips back on.

Type of Change

• Add - New feature or capability
• Change - Changes in existing functionality
• Fix - Bug fixes
• Remove - Removed features or deprecated functionality
• Internal - Internal changes (refactoring, tests, docs, etc.)

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• No testing required (docs, internal refactor, etc.)

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