Honor MountOption.mount_destination on session creation (unified path)

## Objective

Mark MountOption.mount_destination as deprecated for session creation paths and make the description explicit that the field is honored only by the inference service creation wire schema. Session creation runtime ignores it; destinations must come via mount_map / mount_id_map.

## Background

mount_destination was originally added to MountOptionModel in PR #2168 (May 2024) only for the inference service creation wire schema, where extra_mounts is a single dict[UUID, MountOption] without a separate mount_map. Session creation has always used mount_map / mount_id_map for destinations and never read mount_destination from mount_options[uuid]. After the recent unification (PR #11608), the unified MountOption carries mount_destination, but on session paths it is silently ignored. This is a footgun for callers.

## Acceptance Criteria

- MountOption.mount_destination is marked deprecated via Field(deprecated=...)
- Field description explicitly says it applies only to inference service creation
- Service creation REST/SDK flow continues to work unchanged

## Related

- Follow-up to BA-6040 / PR #11608

JIRA Issue: BA-6041