FastMCP masks unexpected exceptions and logs the traceback server-side,
so paths and provider URLs never cross the transport. Expected failures
raise ToolError with a message: unknown document, unknown collection,
invalid filter, invalid base64, and agent failures naming only the
exception type. No tool returns an empty value or an error string on
failure any more.
A filter is validated on its own before the read that would use it, with
a filtered count on one of the selected databases: that is the query
engine rejecting the filter and nothing else, so its message (columns and
the statement) can be forwarded, while a ValueError raised later in the
read stays masked and no database outside the selection is opened.
Refs #599
2a6d72171d