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>

tests/test_unit.py

@@ -31,9 +31,59 @@ bf = _module("beancount_format")
 au = _module("account_utils")
 val = _module("core.validation")
 mdl = _module("models")
+fc = _module("fava_client")
 AccountType = mdl.AccountType
 
 
+# ---------------------------------------------------------------------------
+# fava_client._open_directive_exists — duplicate-account detection
+# ---------------------------------------------------------------------------
+
+
+def test_open_directive_exists_matches_real_directive():
+    src = "2020-01-01 open Expenses:Vehicle:Gas\n"
+    assert fc._open_directive_exists(src, "Expenses:Vehicle:Gas") is True
+
+
+def test_open_directive_exists_matches_currency_constrained_and_metadata():
+    src = (
+        "2020-01-01 open Expenses:Vehicle:Gas EUR, SATS\n"
+        '  added_by: "abc"\n'
+    )
+    assert fc._open_directive_exists(src, "Expenses:Vehicle:Gas") is True
+
+
+def test_open_directive_exists_matches_inline_comment_without_space():
+    # Valid Beancount: the account token ends at ';'. The old (?:\\s|$) boundary
+    # missed this → duplicate Open written → bean-check breaks.
+    src = "2020-01-01 open Expenses:Vehicle:Gas;legacy-import\n"
+    assert fc._open_directive_exists(src, "Expenses:Vehicle:Gas") is True
+
+
+def test_open_directive_exists_ignores_name_inside_description():
+    # The name appears only inside another account's description metadata.
+    src = (
+        "2020-01-01 open Expenses:Notes\n"
+        '  description: "remember to open Expenses:Vehicle:Gas next month"\n'
+    )
+    assert fc._open_directive_exists(src, "Expenses:Vehicle:Gas") is False
+
+
+def test_open_directive_exists_ignores_comment_line():
+    src = "; TODO: open Expenses:Vehicle:Gas eventually\n"
+    assert fc._open_directive_exists(src, "Expenses:Vehicle:Gas") is False
+
+
+def test_open_directive_exists_does_not_match_longer_sibling():
+    src = "2020-01-01 open Expenses:Vehicle:GasStation\n"
+    assert fc._open_directive_exists(src, "Expenses:Vehicle:Gas") is False
+
+
+def test_open_directive_exists_does_not_match_deeper_child():
+    src = "2020-01-01 open Expenses:Vehicle:Gas:Premium\n"
+    assert fc._open_directive_exists(src, "Expenses:Vehicle:Gas") is False
+
+
 # ---------------------------------------------------------------------------
 # beancount_format.sanitize_link
 # ---------------------------------------------------------------------------