fix(recipes): reject the empty-hash placeholder when resolving LoRA hashes

The SHA256 of an empty byte string (written by repackaging tools into
safetensors metadata, or produced by hashing an empty/unreadable file)
was previously resolved against CivitAI's by-hash API, which can contain
polluted entries for it (e.g. a broken SD 1.5 LoRA whose AutoV3 equals
the placeholder) and falsely attributed the wrong model to a recipe.

Guard all lookup paths for the 10/12/64-char AutoV2/AutoV3/full-SHA256
spellings: CivitaiClient.get_model_by_hash/_fetch_version_by_hash return
not-found without a request, and ModelHashIndex ignores the placeholder
in has_hash/get_path/add_autov3.

The Automatic1111 metadata parser keeps the LoRA item itself when its
hash is the placeholder: it matches by filename locally, or retains the
entry with an empty hash flagged hashInvalid (unresolvable-hash state in
the UI, with reconnect as the remedy) instead of dropping it or resolving
it to a polluted CivitAI entry.

tests/utils/test_constants.py

@@ -0,0 +1,45 @@
+"""Tests for the empty-hash placeholder predicate in constants."""
+
+from py.utils.constants import (
+    EMPTY_HASH_SHA256,
+    INVALID_AUTOV2_EMPTY_HASH,
+    INVALID_AUTOV3_EMPTY_HASH,
+    is_empty_placeholder_hash,
+)
+
+
+class TestIsEmptyPlaceholderHash:
+    def test_full_length_sha256(self):
+        assert is_empty_placeholder_hash(EMPTY_HASH_SHA256)
+
+    def test_autov3_length(self):
+        assert is_empty_placeholder_hash("e3b0c44298fc")
+
+    def test_autov2_length(self):
+        assert is_empty_placeholder_hash("e3b0c44298")
+
+    def test_case_insensitive(self):
+        assert is_empty_placeholder_hash("E3B0C44298FC")
+        assert is_empty_placeholder_hash(EMPTY_HASH_SHA256.upper())
+
+    def test_derived_constants_are_prefixes(self):
+        assert INVALID_AUTOV2_EMPTY_HASH == EMPTY_HASH_SHA256[:10]
+        assert INVALID_AUTOV3_EMPTY_HASH == EMPTY_HASH_SHA256[:12]
+
+    def test_rejects_other_lengths(self):
+        # 8-char AutoV1-style prefix and non-placeholder lengths are not it
+        assert not is_empty_placeholder_hash("e3b0c442")
+        assert not is_empty_placeholder_hash("e3b0c44298fc1c")
+        assert not is_empty_placeholder_hash("")
+
+    def test_rejects_real_hashes_that_share_the_prefix(self):
+        # A real hash whose first characters coincide must not be rejected
+        assert not is_empty_placeholder_hash("e3b0c44298aa")
+        assert not is_empty_placeholder_hash("e3b0c44298fc" + "a" * 52)
+        assert not is_empty_placeholder_hash("915a9a1f5f")
+        assert not is_empty_placeholder_hash("915a9a1f5f58")
+        assert not is_empty_placeholder_hash("a" * 64)
+
+    def test_rejects_non_strings(self):
+        assert not is_empty_placeholder_hash(None)
+        assert not is_empty_placeholder_hash(123)