[Rust-dataplane] SHOW SESSION_STATES current-db non-string/null must be ignored like Go #202

Rust dataplane rewrite parity/functional RED

• Snapshot commit: 60efdf5e6e8226423df85033534ad748ccb129db
• Run: run.tiproxy-rust-dataplane.session-states-current-db-type.20260902.RED

Problem

Go updateAuthInfoFromSessionStates only applies current-db when type assertion to string succeeds; null/number is ignored. Rust validate_session_states rejects non-string values as MalformedSessionStates.

Claim

when SHOW SESSION_STATES returns current-db:null or current-db:, Rust must treat it as no authoritative database and continue, matching Go.

Source anchors

• pkg/proxy/backend/backend_conn_mgr.go:729-739 updateAuthInfoFromSessionStates type assertion
• rust/crates/session-core/src/internal_client.rs:757-769 validate_session_states rejects non-string

Boundary

• Before: Rust rejects the whole snapshot as MalformedSessionStates
• After: Go ignores non-string current-db and proceeds with current_database None

Local evidence

• run.tiproxy-rust-dataplane.session-states-current-db-type.20260902.RED