Sanitizes reference links for Beancount

Ensures that user-provided reference strings for expense,
receivable, and revenue entries are sanitized before being
included as Beancount links. This prevents issues caused by
invalid characters in the links, improving compatibility with
Beancount's link format. A new utility function is introduced
to handle the sanitization process.

beancount_format.py

@@ -14,6 +14,31 @@ Key concepts:
 from datetime import date, datetime
 from decimal import Decimal
 from typing import Any, Dict, List, Optional
+import re
+
+
+def sanitize_link(text: str) -> str:
+    """
+    Sanitize a string to make it valid for Beancount links.
+
+    Beancount links can only contain: A-Z, a-z, 0-9, -, _, /, .
+    All other characters are replaced with hyphens.
+
+    Examples:
+        >>> sanitize_link("Test (pending)")
+        'Test-pending'
+        >>> sanitize_link("Invoice #123")
+        'Invoice-123'
+        >>> sanitize_link("castle-abc123")
+        'castle-abc123'
+    """
+    # Replace any character that's not alphanumeric, dash, underscore, slash, or period with a hyphen
+    sanitized = re.sub(r'[^A-Za-z0-9\-_/.]', '-', text)
+    # Remove consecutive hyphens
+    sanitized = re.sub(r'-+', '-', sanitized)
+    # Remove leading/trailing hyphens
+    sanitized = sanitized.strip('-')
+    return sanitized
 
 
 def format_transaction(