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/client.py

@@ -306,3 +306,79 @@ async def create_missing_datasets(
     ) as client:
         for path in paths:
             await create_dataset(client, path)
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# iSCSI zvol utilities
+# ─────────────────────────────────────────────────────────────────────────────
+
+async def check_iscsi_zvols(
+    client: TrueNASClient,
+    zvol_names: list[str],
+) -> list[str]:
+    """
+    Return the subset of *zvol_names* that do not exist on the destination.
+    Names are the dataset path without the leading 'zvol/' prefix
+    (e.g. 'tank/VMWARE001').  Returns [] when the query itself fails.
+    """
+    if not zvol_names:
+        return []
+
+    unique = sorted(set(zvol_names))
+    log.info("Checking %d zvol(s) against destination datasets …", len(unique))
+    try:
+        datasets = await client.call(
+            "pool.dataset.query", [[["type", "=", "VOLUME"]]]
+        ) or []
+    except RuntimeError as exc:
+        log.warning("Could not query zvols (skipping check): %s", exc)
+        return []
+
+    existing = {d["name"] for d in datasets}
+    missing = [n for n in unique if n not in existing]
+    if missing:
+        for n in missing:
+            log.warning("  MISSING zvol: %s", n)
+    else:
+        log.info("  All iSCSI zvols exist on destination.")
+    return missing
+
+
+async def create_zvol(
+    client: TrueNASClient,
+    name: str,
+    volsize: int,
+) -> bool:
+    """
+    Create a ZFS volume (zvol) on the destination.
+    *name* is the dataset path (e.g. 'tank/VMWARE001').
+    *volsize* is the size in bytes.
+    Returns True on success, False on failure.
+    """
+    log.info("Creating zvol %r (%d bytes) …", name, volsize)
+    try:
+        await client.call("pool.dataset.create", [{
+            "name":    name,
+            "type":    "VOLUME",
+            "volsize": volsize,
+        }])
+        log.info("  Created: %s", name)
+        return True
+    except RuntimeError as exc:
+        log.error("  Failed to create zvol %r: %s", name, exc)
+        return False
+
+
+async def create_missing_zvols(
+    host: str,
+    port: int,
+    api_key: str,
+    zvols: dict[str, int],
+    verify_ssl: bool = False,
+) -> None:
+    """Open a fresh connection and create zvols from {name: volsize_bytes}."""
+    async with TrueNASClient(
+        host=host, port=port, api_key=api_key, verify_ssl=verify_ssl,
+    ) as client:
+        for name, volsize in zvols.items():
+            await create_zvol(client, name, volsize)