feat: experiment log stream, to file and UI

Adds a few new logging utility classes. One to save to files with a date, one to support optional fields in formats, last to filter partial log messages.

ref: N25B-401

src/control_backend/logging/optional_field_formatter.py

@@ -0,0 +1,67 @@
+import logging
+import re
+
+
+class OptionalFieldFormatter(logging.Formatter):
+    """
+    Logging formatter that supports optional fields marked by `?`.
+
+    Optional fields are denoted by placing a `?` after the field name inside
+    the parentheses, e.g., `%(role?)s`. If the field is not provided in the
+    log call's `extra` dict, it will use the default value from `defaults`
+    or `None` if no default is specified.
+
+    :param fmt: Format string with optional `%(name?)s` style fields.
+    :type fmt: str or None
+    :param datefmt: Date format string, passed to parent :class:`logging.Formatter`.
+    :type datefmt: str or None
+    :param style: Formatting style, must be '%'. Passed to parent.
+    :type style: str
+    :param defaults: Default values for optional fields when not provided.
+    :type defaults: dict or None
+
+    :example:
+
+        >>> formatter = OptionalFieldFormatter(
+        ...     fmt="%(asctime)s %(levelname)s %(role?)s %(message)s",
+        ...     defaults={"role": ""-""}
+        ... )
+        >>> handler = logging.StreamHandler()
+        >>> handler.setFormatter(formatter)
+        >>> logger = logging.getLogger(__name__)
+        >>> logger.addHandler(handler)
+        >>>
+        >>> logger.chat("Hello there!", extra={"role": "USER"})
+        2025-01-15 10:30:00 CHAT USER Hello there!
+        >>>
+        >>> logger.info("A logging message")
+        2025-01-15 10:30:01 INFO - A logging message
+
+    .. note::
+        Only `%`-style formatting is supported. The `{` and `$` styles are not
+        compatible with this formatter.
+
+    .. seealso::
+        :class:`logging.Formatter` for base formatter documentation.
+    """
+
+    # Match %(name?)s or %(name?)d etc.
+    OPTIONAL_PATTERN = re.compile(r"%\((\w+)\?\)([sdifFeEgGxXocrba%])")
+
+    def __init__(self, fmt=None, datefmt=None, style="%", defaults=None):
+        self.defaults = defaults or {}
+
+        self.optional_fields = set(self.OPTIONAL_PATTERN.findall(fmt or ""))
+
+        # Convert %(name?)s to %(name)s for standard formatting
+        normalized_fmt = self.OPTIONAL_PATTERN.sub(r"%(\1)\2", fmt or "")
+
+        super().__init__(normalized_fmt, datefmt, style)
+
+    def format(self, record):
+        for field, _ in self.optional_fields:
+            if not hasattr(record, field):
+                default = self.defaults.get(field, None)
+                setattr(record, field, default)
+
+        return super().format(record)