fix(accounts): anchor duplicate-account detection to a real Open directive (libra-#48)

The existence check matched 'open <name>' anywhere in the chart source,
so a prior account's description metadata or a comment mentioning the
name produced a false 409, while a real directive with an inline comment
and no space ('open X;legacy') was missed → a duplicate Open was appended
and bean-check then rejected the file, breaking every later /api/source
write. Extract the check into a pure _open_directive_exists() anchored to
'^YYYY-MM-DD open <name>' with an account-boundary negative-lookahead, and
unit-test both failure directions plus prefix/child non-matches.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

fava_client.py

@@ -60,6 +60,30 @@ def _escape_beancount_string(value: str) -> str:
     )
 
 
+def _open_directive_exists(source: str, account_name: str) -> bool:
+    """Return True if `source` already contains an Open directive for exactly
+    `account_name`.
+
+    Anchored to a real `YYYY-MM-DD open <account>` directive line (re.MULTILINE)
+    so the account name can't match text inside another account's description
+    metadata or a comment (false positive → spurious 409). The trailing
+    negative-lookahead `(?![\\w:-])` requires the next char not to be an
+    account-continuation char, so:
+      - a prefix (Expenses:Gas) does not match a longer sibling
+        (Expenses:GasStation / Expenses:Gas:Vehicle), and
+      - a real directive with an inline comment and no space
+        (`open Expenses:Gas;legacy`) is still detected (`;` ends the name),
+        which the previous `(?:\\s|$)` boundary missed → duplicate write.
+    """
+    return bool(
+        re.search(
+            rf"^\d{{4}}-\d{{2}}-\d{{2}} open {re.escape(account_name)}(?![\w:-])",
+            source,
+            re.MULTILINE,
+        )
+    )
+
+
 class FavaClient:
     """
     Async client for Fava REST API.
@@ -1644,15 +1668,9 @@ class FavaClient:
                         source = source_data["source"]
 
                         # Step 2: Check if account already exists (may have been
-                        # created by a concurrent request). Anchor on the Open
-                        # directive and require the account to be followed by
-                        # whitespace/end-of-line so a prefix (Expenses:Gas) does
-                        # not match a longer sibling (Expenses:GasStation).
-                        if re.search(
-                            rf"open {re.escape(account_name)}(?:\s|$)",
-                            source,
-                            re.MULTILINE,
-                        ):
+                        # created by a concurrent request). See
+                        # _open_directive_exists for the anchoring rationale.
+                        if _open_directive_exists(source, account_name):
                             logger.info(f"Account {account_name} already exists in {target_file}")
                             return {
                                 "data": sha256sum,