fix(calendar): redact script diagnostics, and page the calendar list

Three findings from CodeRabbit, all valid.

Raw subprocess output was being returned to the client -- the script's
stderr on one path, and its own error payload on another. CodeQL flagged
the same line. That script handles OAuth client secrets and interpolates
exceptions into its messages, so either could carry a secret or a path.
Both now go to the log unredacted, where they are worth having in full,
and reach the client through a redactor.

That redactor already existed inside describe_exception, which only
takes exceptions. Split out as redact_text: an exception is not the only
thing worth returning, and a subprocess's stderr is just as capable of
quoting a token.

calendarList.list returns 100 entries per page by default, caps at 250,
and hands back a nextPageToken when there are more. Reading one page
would have hidden calendars from the picker with nothing to say the list
was cut short. It now pages, asking for 250 at a time, bounded at ten
pages so a malformed token cannot spin.

And a test helper was a lambda where ruff wants a def.

The five new tests fail against the previous commit.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Udr6MfaFLUPhX5Fgo67Jf5

src/web_interface/error_handler.py

@@ -77,6 +77,25 @@ def describe_exception(exc: BaseException,
     """
     message = str(exc).strip()
     text = f"{type(exc).__name__}: {message}" if message else type(exc).__name__
+    return redact_text(text, max_length)
+
+
+def redact_text(text: str, max_length: int = _MAX_DETAIL_LENGTH) -> str:
+    """Make arbitrary text safe to hand back over HTTP.
+
+    Split out of describe_exception because exceptions are not the only thing
+    worth returning: a subprocess's stderr, or a message a helper script
+    printed, is just as useful to a user and just as capable of carrying a
+    token or a password in it.
+
+    Args:
+        text: The text to redact
+        max_length: Truncate beyond this many characters
+
+    Returns:
+        A single line, credentials replaced, length capped.
+    """
+    text = text or ''
     # Order matters: the URL and header forms are more specific than the
     # generic key=value pattern, which would otherwise chew the scheme.
     text = _REDACT_URL_USERINFO.sub(r'\1<redacted>\3', text)