Strip leading UTF-8 BOM in client transaction-tracker classifier helper

Symmetric with the dbapi-side fix. Python's str.strip() does not
consider U+FEFF whitespace, so a BOM-prefixed BEGIN / COMMIT /
ROLLBACK / SAVEPOINT / RELEASE silently desynced the transaction
tracker from the server. SQLite's sqlite3_prepare_v2 skips a leading
BOM before tokenisation, so the fix restores SQLite parity.

Add a leading lstrip(U+FEFF) before the existing strip(); a parity
test in the dbapi tree pins both copies.

Author: antoineleclair

src/dqliteclient/connection.py

@@ -243,8 +243,15 @@ def _strip_leading_comments(sql: str) -> str:
     step with the server. This helper is duplicated from the dbapi
     cursor's identical helper rather than introducing an inter-package
     import — the parser is small and stable.
+
+    Also strips a leading UTF-8 BOM (``\\ufeff``) for parity with
+    SQLite's ``sqlite3_prepare_v2``, which silently skips it. Python's
+    ``str.strip()`` does NOT consider ``\\ufeff`` whitespace, so a SQL
+    file imported via ``encoding='utf-8'`` (instead of ``utf-8-sig``)
+    or written by PowerShell ``Set-Content`` / Notepad would otherwise
+    desync the savepoint / transaction tracker.
     """
-    s = sql.strip()
+    s = sql.lstrip("").strip()
     while True:
         if s.startswith("--"):
             newline = s.find("\n")

tests/test_strip_leading_comments_bom.py

@@ -0,0 +1,33 @@
+"""Pin: client-layer ``_strip_leading_comments`` strips leading
+UTF-8 BOM. See dbapi-side test for full rationale.
+"""
+
+from __future__ import annotations
+
+from dqliteclient.connection import _starts_with_tx_verb, _strip_leading_comments
+
+_BOM = ""
+
+
+def test_strip_leading_comments_strips_lone_bom() -> None:
+    assert _strip_leading_comments(f"{_BOM}BEGIN") == "BEGIN"
+
+
+def test_strip_leading_comments_strips_bom_then_whitespace() -> None:
+    assert _strip_leading_comments(f"{_BOM}   COMMIT") == "COMMIT"
+
+
+def test_strip_leading_comments_strips_bom_then_line_comment() -> None:
+    assert _strip_leading_comments(f"{_BOM}-- hi\nROLLBACK") == "ROLLBACK"
+
+
+def test_strip_leading_comments_strips_bom_then_block_comment() -> None:
+    assert _strip_leading_comments(f"{_BOM}/* hi */ SAVEPOINT a") == "SAVEPOINT a"
+
+
+def test_starts_with_tx_verb_recognises_bom_prefixed_begin() -> None:
+    assert _starts_with_tx_verb(f"{_BOM}BEGIN") is True
+
+
+def test_starts_with_tx_verb_recognises_bom_prefixed_commit() -> None:
+    assert _starts_with_tx_verb(f"{_BOM}COMMIT") is True