Read a numeric natural key as a name before reading it as an id

An Azure device import died part-way with a foreign-key error, and the crash
was the lucky outcome.

The Azure adapter routes a device that already exists to its real asset by
writing that asset's assetnumber into the assetid column of the CSV it
generates, expecting the engine to look it up. The resolver instead takes any
all-digit string at face value as a literal id. This site numbers operations
0NNN, so numeric assetnumbers are ordinary here: `0704` was read as id 704,
which does not exist, hence the error. `0600` would have been read as id 600 -
a printer - and bound a PC to it silently, reporting success.

Two changes, both needed.

The resolver now tries the label first for an all-digit string and only falls
back to reading it as an id when nothing is named that. The face-value
behaviour exists for a real reason - exports round-trip and carry real ids, as
its original comment says - so it stays as the fallback rather than being
removed. Somebody writing `0600` in a column keyed on assetnumber means the
asset called 0600, and binding the row to an unrelated record is far worse than
failing.

_existing_hostname_map now hands over the assetid rather than the assetnumber.
Giving the resolver the id it is going to treat the value as makes the
passthrough correct by construction instead of correct by luck.

Verified against a restored copy of the production database: 234 devices, 221
matching an existing computer and updating in place, 13 new, no problems. The
same run previously failed on the foreign key.

Neither half was wrong alone. The passthrough dates from the original CSV
import and the hostname map from the wilm merge months later; they only meet
at a site whose natural keys are numeric.

Scope: product

tests/test_csv_import.py

@@ -335,3 +335,29 @@ def test_reconcile_still_binds_normalized_exact(db):
     _db.session.rollback()
 
 
+
+
+def test_a_numeric_assetnumber_is_read_as_a_name_not_an_id(db):
+    """This site numbers operations 0NNN, so numeric natural keys are real.
+
+    An all-digit string used to be taken at face value as an id. `0600` became
+    id 600 - a different asset, or none - which is how an Azure device import
+    died on a foreign key partway through, and how a PC could instead have been
+    bound silently to a printer's record. The label wins when one matches.
+    """
+    resolver = Resolver()
+    seed_reference(resolver)
+    import_csv('assets', 'assetnumber,assettypeid\n0600,Machine\n',
+               resolver=resolver, commit=True)
+    realid = _db.session.execute(_db.text(
+        "select assetid from assets where assetnumber = '0600'")).scalar()
+
+    # Force the ambiguity: an id that is NOT the one named 0600.
+    assert resolver.resolve('assets', 'assetid', '0600') == realid
+
+
+def test_a_number_naming_nothing_is_still_read_as_an_id(db):
+    """Round-tripped exports and sites that know their ids keep working."""
+    resolver = Resolver()
+    seed_reference(resolver)
+    assert resolver.resolve('assets', 'assetid', '999999') == 999999