chore(llm-routing): reject deprecated multiregion routing aliases at input

Description

Follow-up to #536 (per-model routing configuration). Non-blocking.

Reject the deprecated multiregion routing method spellings at input in the
function API and the CLI, with an error naming the canonical replacements.
Write-time normalization already keeps newly persisted data canonical, so this
is an input-vocabulary cleanup, not a data change. The router keeps read-path
compatibility for pre-existing persisted rows, which are not migrated.

Definition of Done

• API and CLI reject the deprecated multiregion spellings on create and
  update with an error naming the canonical names
• Existing persisted values keep routing unchanged
• Public docs list only canonical names

Resources

• Parent: #536
• Blocked by: none (can land any time after the parent's control-plane ticket)