fix(mysql): remove path-separator check from MysqlAgentStateStore session id validation (#2022)

## Summary
MysqlAgentStateStore.validateSessionId() rejects any session id
containing / or \, which makes it incompatible with legitimate callers
such as SessionSandboxStateStore. This
PR removes that check because it is a filesystem-oriented constraint
that has no meaning for a relational store and violates the
AgentStateStore interface contract that other
implementations (e.g. JsonFileAgentStateStore) already honor.