Add --dry-run flag to mkdir

[AndreyVMarkelov]

Summary

Extends dry-run support to dbxcli mkdir, reusing the shared helpers introduced with rm --dry-run. With --dry-run, mkdir previews the intended folder creation without calling CreateFolderV2: it reports a planned JSON status and prints Would create directory <path> in text mode.

This is the second command to adopt the shared cmd/dry_run.go helpers, which validates the abstraction. It also includes two small cleanups:

• Extract dryRunDisplayPath — the duplicated displayPath logic (present-or-fallback) is hoisted into a shared helper now used by both rm and mkdir.
• Reword the flag help to Preview intended writes without making changes, which better reflects that dry-run previews intent rather than exact outcome (mkdir's offline preview reports planned even where a real run would report existing).

The help manifest, JSON contract, input/definition schemas, and docs are updated to include the dry_run input and planned status for mkdir.

Design note

mkdir --dry-run is a fully offline preview — it calls neither CreateFolderV2 nor GetMetadata (tests assert both). This is intentional: mkdir's execute path is tangled with conflict handling and doesn't cleanly split into validate/skip like rm does. The reworded flag help frames the contract as "preview intended writes," which both commands satisfy honestly.