Switch artifact update DTO to Unset

## Scope

|Target|Change|
|---|---|
|DTO|`common/dto/manager/v2/artifact/request.py` — 1 field|
|Adapter|`manager/api/adapters/artifact/adapter.py` — 1 branch|
|CLI|`client/cli/v2/artifact/commands.py` L60-62 — `SENTINEL` → `UNSET`|
|Tests|`tests/unit/common/dto/manager/v2/artifact/test_request.py` and any other `SENTINEL` references|

## 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/.enhance.md`

## Done when

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

JIRA Issue: BA-7745

Start with common/dto/manager/v2/artifact/request.py, manager/api/adapters/artifact/adapter.py, and client/cli/v2/artifact/commands.py lines 60-62, then inspect the named unit tests and other SENTINEL references. Run the artifact update command to verify omitted fields remain unchanged, null clears nullable fields, and values update fields; confirm no Sentinel or SENTINEL references remain in this domain.

Written by the indexing model from the issue text.