Fix test failures: update hash test for MD5, handle unicode surrogates

- Update test_hash_uses_sha256_truncated → test_hash_uses_md5_truncated
  to match the SHA256→MD5 change in compression_store.py
- Use errors="surrogatepass" in compute_hash to handle lone surrogates
  in unicode content (fixes pre-existing UnicodeEncodeError)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

headroom/utils.py

@@ -23,7 +23,7 @@ def generate_request_id() -> str:
 def compute_hash(data: str | bytes) -> str:
     """Compute SHA256 hash, returning hex string."""
     if isinstance(data, str):
-        data = data.encode("utf-8")
+        data = data.encode("utf-8", errors="surrogatepass")
     return hashlib.sha256(data).hexdigest()
 
 

tests/test_compression_store.py

@@ -1249,10 +1249,10 @@ class TestHashCollisionDetection:
         # Should not have collision warning
         assert "Hash collision detected" not in caplog.text
 
-    def test_hash_uses_sha256_truncated(self, store: CompressionStore):
-        """Hash is SHA256 truncated to 24 characters."""
+    def test_hash_uses_md5_truncated(self, store: CompressionStore):
+        """Hash is MD5 truncated to 24 characters (fast, non-crypto)."""
         content = "test content"
-        expected_hash = hashlib.sha256(content.encode()).hexdigest()[:24]
+        expected_hash = hashlib.md5(content.encode()).hexdigest()[:24]
 
         hash_key = store.store(original=content, compressed="[]")