Use the vendor's empty spool weight as tare in Spoolman mode (#3195)

Spoolman resolves a spool's tare from the spool, then the filament, then
the vendor's empty_spool_weight. Bambuddy skipped the vendor and fell
back to 250 g, so weighing a spool whose tare was set only on its vendor
gave the wrong remaining weight. The inventory weigh action, the
SpoolBuddy scale and the displayed core weight now share one lookup, and
"keep old weight" on a filament change stamps a vendor-inherited tare.

backend/tests/unit/test_spoolman_inventory_helpers.py

@@ -8,6 +8,7 @@ from backend.app.api.routes._spoolman_helpers import (
     _map_spoolman_spool,
     _safe_float,
     _safe_int,
+    spoolman_tare,
 )
 
 # ---------------------------------------------------------------------------
@@ -466,6 +467,56 @@ class TestMapSpoolmanSpool:
         spool = {**MINIMAL_SPOOL, "spool_weight": None, "filament": {**MINIMAL_SPOOL["filament"], "spool_weight": None}}
         assert _map_spoolman_spool(spool)["core_weight"] == 250
 
+    def test_core_weight_from_vendor_empty_spool_weight(self):
+        """Spoolman's third tare level is the vendor's empty_spool_weight (#3195)."""
+        spool = {
+            **MINIMAL_SPOOL,
+            "spool_weight": None,
+            "filament": {**MINIMAL_SPOOL["filament"], "spool_weight": None, "vendor": {"empty_spool_weight": 211.7}},
+        }
+        result = _map_spoolman_spool(spool)
+        assert result["core_weight"] == 211
+        # Still inherited: the spool has no tare of its own, so the form must not stamp it.
+        assert result["core_weight_is_inherited"] is True
+
+
+class TestSpoolmanTare:
+    """spoolman_tare resolves the tare the way Spoolman's /measure does (#3195)."""
+
+    @staticmethod
+    def _spool(spool_level=None, filament_level=None, vendor_level=None, vendor=True):
+        filament: dict = {"spool_weight": filament_level}
+        if vendor:
+            filament["vendor"] = {"id": 5, "name": "Sunlu", "empty_spool_weight": vendor_level}
+        return {"id": 1, "spool_weight": spool_level, "filament": filament}
+
+    @pytest.mark.parametrize(
+        ("levels", "expected"),
+        [
+            ((300.0, 180.0, 211.7), (300.0, "spool")),
+            ((None, 180.0, 211.7), (180.0, "filament")),
+            ((None, None, 211.7), (211.7, "vendor")),
+            ((None, None, None), (250.0, "fallback")),
+        ],
+    )
+    def test_resolution_order(self, levels, expected):
+        assert spoolman_tare(self._spool(*levels)) == expected
+
+    @pytest.mark.parametrize("level", ["spool", "filament", "vendor"])
+    def test_zero_is_a_real_tare_at_every_level(self, level):
+        levels = {"spool": (0, 180.0, 211.7), "filament": (None, 0, 211.7), "vendor": (None, None, 0)}[level]
+        assert spoolman_tare(self._spool(*levels)) == (0.0, level)
+
+    @pytest.mark.parametrize("bad", [float("nan"), float("inf"), "abc"])
+    def test_non_finite_value_counts_as_missing(self, bad):
+        assert spoolman_tare(self._spool(bad, None, 211.7)) == (211.7, "vendor")
+
+    def test_missing_vendor_null_vendor_and_null_filament(self):
+        assert spoolman_tare(self._spool(vendor=False)) == (250.0, "fallback")
+        assert spoolman_tare({"id": 1, "filament": {"vendor": None}}) == (250.0, "fallback")
+        assert spoolman_tare({"id": 1, "filament": None}) == (250.0, "fallback")
+        assert spoolman_tare({"id": 1}) == (250.0, "fallback")
+
 
 # ---------------------------------------------------------------------------
 # F4: _safe_optional_float unit tests