Switch idle_checker update DTO to Unset

[wngh4628]

Scope

Target	Change
DTO	common/dto/manager/v2/idle_checker/request.py — 1 field
Adapter	None here (the idle_checker adapter branch is covered by the prometheus_query_preset story)
CLI	client/cli/v2/admin/idle_checker.py L209 — SENTINEL → UNSET

Common work

• DTO: X | Sentinel | None = Field(default=SENTINEL) → X | None | Unset = Field(default=UNSET), importing from ai.backend.common.tristate.unset
• Adapter: TriState.from_unset for nullable columns, OptionalState.from_unset for non-nullable columns
• Unify Field(description=...) wording to "Omit to leave unchanged; null clears." (drop "null clears" for non-nullable fields)
• Tests: SENTINEL → UNSET, Sentinel → Unset
• News fragment changes/<PR#>.enhance.md

Done when

• Zero Sentinel / SENTINEL references remain in this domain
• Verified with ./bai admin idle-checker update: field omitted → unchanged, null → cleared, value → updated

JIRA Issue: BA-7747