Add zvol existence check and creation for iSCSI extents

client.py:
- check_iscsi_zvols(): queries pool.dataset.query for VOLUME type,
  returns list of missing zvol names
- create_zvol(): creates a single zvol via pool.dataset.create
- create_missing_zvols(): opens a fresh connection and creates a
  batch of zvols from a {name: volsize_bytes} dict

summary.py:
- Add zvols_to_check and missing_zvols list fields
- Report shows a WARNING block listing missing zvols when present

migrate.py:
- _migrate_iscsi_extents() populates summary.zvols_to_check with
  the dataset name for each DISK-type extent during dry run

cli.py:
- Add _parse_size() to parse human-friendly size strings
  (100G, 500GiB, 1T, etc.) to bytes
- run() calls check_iscsi_zvols() during dry run and stores results
  in summary.missing_zvols
- Wizard prompts for size and creates missing zvols after the dry
  run report, before asking the user to confirm the live run

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

truenas_migrate/migrate.py

@@ -236,6 +236,8 @@ async def _migrate_iscsi_extents(
                      _cyan("[DRY RUN]"), _bold_cyan(repr(name)),
                      ext.get("disk") or ext.get("path"))
             summary.iscsi_extents_created += 1
+            if ext.get("type") == "DISK" and ext.get("disk"):
+                summary.zvols_to_check.append(ext["disk"].removeprefix("zvol/"))
             continue
 
         try: