Sets the per-session DLM boost flag for an active session.

When enabled, the boosted EVSE receives the unused headroom between
the active circuit constraint (schedule limit and/or flexibility
request) and the circuit's maximum capacity, capped at the EVSE's
maximum supported current. The change applies on the next DLM
algorithm tick.

The flag persists for the remainder of the session. The boost ends
when either the session ends (no explicit call required) or the
client sends this action again with enabled: false.

Sending the same value the session already carries is accepted as
an idempotent no-op (returns 202 with no second effect).

The action is rejected with HTTP 422 if the session is not in the
active state.