[Rust-dataplane] COM_SET_OPTION MULTI_STATEMENTS changes must propagate to Rust backend/migration handshake masks #204

Rust dataplane rewrite parity/functional RED

• Snapshot commit: 60efdf5e6e8226423df85033534ad748ccb129db
• Run: run.tiproxy-rust-dataplane.set-option-capability-propagation.20260902.RED

Problem

Go updates Authenticator.capability and CmdProcessor.capability after successful COM_SET_OPTION. Rust only updates CommandSessionState.capabilities; session_engine.self.negotiated is never updated and cmd_state.capabilities() is not used by any production path. Backend/migration/change-user handshakes keep using the stale original negotiated mask.

Claim

after successful COM_SET_OPTION toggles CLIENT_MULTI_STATEMENTS, Rust should use the updated capability for later backend, migration, and change-user handshakes as Go does.

Source anchors

• pkg/proxy/backend/backend_conn_mgr.go ComSetOption updates authenticator.capability
• rust/crates/session-core/src/command.rs SessionMutation::SetMultiStatements
• rust/crates/dataplane/src/session_engine.rs apply_command_mutations only cmd_state
• rust/crates/dataplane/src/session_engine.rs plan_backend_migration_handshake(self.negotiated,...)

Boundary

• Before: Rust backend/migration handshakes use original self.negotiated even after COM_SET_OPTION
• After: Rust propagates cmd_state capability changes into handshake planners/encoders

Local evidence

• run.tiproxy-rust-dataplane.set-option-capability-propagation.20260902.RED