feat: add restored session manager support and harden SSE startup

[0xCaso]

Summary

• add SessionManager restore support so callers can resume Tempo session state across process restarts
• export the Tempo client entrypoint and harden SSE startup by preserving Headers instances and failing fast on non-OK SSE responses
• keep restored close behavior consistent by clearing opened state after a successful close

Motivation

• OpenCode-style clients restart frequently, but still need to continue using existing Tempo session channels
• without restore support, callers can only rely on in-memory session state and lose continuity after process restart
• the SSE hardening changes make restored and fresh session flows fail more predictably when the upstream does not actually return a valid stream

Test Plan

• VITE_NODE_ENV=testnet pnpm test --project node src/tempo/client/SessionManager.test.ts src/tempo/client/Session.test.ts
• pnpm run check:types