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 the configured Postgrex JSON library cannot encode - 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 accepted by the configured JSON library is converted to JSON primitives; a rejected struct 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.