Produce a JSON-safe copy of an MCP session-state map before it is written to a jsonb column.

Anubis folds the whole request conn.assigns into the persisted session frame, so the state map an Anubis.Server.Session.Store is handed can carry values with no JSON.Encoder implementation - a request-scoped struct (e.g. a host's authenticated-user struct), a tuple, a pid, a ref, a function, a MapSet. Encoding such a map raises in the database driver, which - because it happens mid-initialize - would crash the session process and drop the connection.

sanitize/1 recursively drops anything non-encodable from its enclosing map or list while preserving encodable siblings:

• a struct with a JSON.Encoder impl (DateTime, Date, Decimal, ...) is kept; a bare struct without one is dropped;
• a map keeps only entries whose key is a valid JSON object key (binary or atom) and whose value survives sanitization;
• a list keeps only its surviving elements.

Keys are otherwise preserved as-is, so a string-keyed restore reads the same values back after the jsonb round-trip. The result always encodes cleanly.