When a bare Slack user ID (U-prefix) is passed as the send target
without an explicit `user:` prefix, `parseSlackTarget` classifies it as
kind="channel". `resolveChannelId` then passes it through to callers
without calling `conversations.open`.
This works for `chat.postMessage` (which tolerates user IDs), but
`files.uploadV2` delegates to `completeUploadExternal` which validates
`channel_id` against `^[CGDZ][A-Z0-9]{8,}$` — rejecting U-prefixed
IDs with `invalid_arguments`.
Fix: detect U-prefixed IDs in `resolveChannelId` regardless of the
parsed `kind`, and always resolve them via `conversations.open` to
obtain the DM channel ID (D-prefix).
Includes test coverage for bare, prefixed, and mention-style user ID
targets with file uploads, plus a channel-target negative case.
af9881b9c5