fix: resolve server panic and auth failure on client connection

[inureyes]

Summary

Fix two critical bugs that caused bssh-server to crash or reject every client connection since v2.0.1:

• Panic on connect: new_client_with_addr called block_on() inside the tokio async runtime to check the auth rate limiter ban list, causing an immediate panic ("Cannot start a runtime from within a runtime"). Added AuthRateLimiter::try_is_banned() using non-blocking RwLock::try_read().

• Auth success → reject: All SshHandler constructors created a local SessionInfo that was never registered with SessionManager. After pubkey/password verification succeeded, authenticate_session() returned SessionNotFound, rejecting the client. Fixed by deferring session creation to the auth flow via SessionManager::create_session().

Test plan

• 429 unit tests pass (21 binary + 408 library)
• 33-item integration test suite with system SSH client — all pass
  • Basic exec (echo, whoami, pwd, uname, date, id, ls)
  • Exit code propagation (0, 1, 42, true, false)
  • Pipes (echo|cat, seq|wc, echo|grep, cat|head)
  • Stdin forwarding
  • Large output (seq 1000, yes 500)
  • Security metachar rejection (&&, ;, $HOME, $(), backtick → exit 126)
  • Nonexistent command (exit 127)
  • Rapid sequential connections (5x)
  • Host key algorithm selection (ed25519, rsa-sha2-256)
• Server survives all tests without crash
• Graceful shutdown (SIGTERM/SIGINT) works
• All subcommands work (version, gen-config, gen-host-key, check-config)