fix(ios): test the SSH tunnel against real OpenSSH servers and name its failures

[datlechin]

Summary

• apple/scripts/ssh-test-servers.sh and scripts/lib/openssh-fixture.sh: pinned OpenSSH 10.3 and 9.9 servers (PQ-only, MAC-only, RSA SHA-2, keyboard-interactive, TOTP, MaxAuthTries, two hops, loss), published on 127.0.0.1 only. sftp-test-server.sh uses the same pin and banner wait.
• apple/scripts/probes/check-ssh-tunnel-matrix.sh: builds the iOS tunnel sources for the simulator against the shipped libssh2 and runs 98 cells through a Redis PING, two-hop chains included. A probe, not CI: no CI job has Docker.
• iOS SSH errors are named instead of "error -N": no common key exchange, host key, cipher or MAC (with the server software and TablePro's list), FIDO key, passphrase missing or wrong, second sign-in step, unanswerable prompt, unsupported auth method.
• iOS password sign-in falls back to keyboard-interactive, answering only prompts that read as a password.
• iOS auth reads the server's method list first. An encrypted key goes without its .pub (unless it is a certificate), so a wrong passphrase fails locally.
• LibSSH2ChannelIO (both apps): a transport error ends the relay as a hangup, not a channel close. An iOS tunnel relaying data now stops at once on a dead server instead of after about 37 s.
• SSHChannelRelay (both apps) sends with MSG_NOSIGNAL: a client closing during channel data no longer kills the process with SIGPIPE. feat(ios): open connections through SSH jump hosts #3683 landed the same send flag; its test and this PR's are one case now.
• Jump hosts (feat(ios): open connections through SSH jump hosts #3683): a named error at a hop comes back as jumpHostFailed, which names the hop and keeps the words. One by-case classifier, ErrorClassifier.sshTunnelFailure, holds the jump host cases and this PR's.
• iOS tunnel: sockets closed after libssh2_session_free; channel free retried until done (SSHChannelFreePump) instead of leaving an unsent CLOSE; channel open polls 100 ms until its 6 s deadline, which fixed 30 clients on one tunnel.
• TableProSSHTransport: SSHNegotiationFailure, SSHPrivateKeyTraits, SSHAuthMethods, KeyboardInteractivePromptKind (the Mac's KBDINTPromptType, moved).
• Docs: docs(ios): sign in to keyboard-interactive SSH servers with a password docs#57.

PQ-only server, before: SSH handshake failed: Handshake failed (error -5). After: TablePro and the SSH server have no key exchange or host key method in common. The server runs OpenSSH_10.3. TablePro supports these key exchange methods: curve25519-sha256, ...

Test plan

• Fail on main, pass here: SSHChannelRelayTests (transport failure, closed client), SSHNegotiationFailureTests, SSHPrivateKeyTraitsTests, SSHAuthMethodsTests, KeyboardInteractivePromptKindTests, SSHChannelFreePumpTests, LibSSH2ChannelIOTests, LibSSH2AuthenticationTests, SSHTunnelErrorClassifierTests.
• SSHTunnelFactoryTests.unsupportedAuthMethodDiscards (Keyboard Interactive since feat(ios): open connections through SSH jump hosts #3683) now expects authMethodUnsupported(.keyboardInteractive). New: namedHopFailureNamesTheHop. KeyboardInteractiveContextTests gains only import TableProSSHTransport.
• apple/scripts/probes/check-ssh-tunnel-matrix.sh on the simulator: 93 of 93 before feat(ios): open connections through SSH jump hosts #3683. After merging it: the two-hop, agent and loss cells 8 of 8 (a killed hop stops a relaying tunnel at once). On main the key, passphrase, FIDO, PQ, MAC and keyboard-interactive cells show only error numbers, and a relaying tunnel takes about 37 s to notice a dead server.
• The faster dead-server notice on the Mac is inferred from the shared code: the matrix has no Mac cells yet.

Part of #3662

Not in this PR

• macOS tunnel cells and the Linux suite additions.
• Connecting to a PQ-only server: libssh2 1.11.1 has no ML-KEM or sntrup, and ML-KEM in libssh2 master needs OpenSSL 3.5.
• A dead-tunnel signal from IOSSSHProvider to the session, as the Mac's onDeath does.
• iOS sets no TCP keepalive, so an idle tunnel waits on the 30 s keep-alive.
• The same teardown, channel free and channel-open wait defects in the macOS tunnel.
• libssh2 1.11.1 ignores the partial-success flag, so a second factor is read from a re-read method list. With MaxAuthTries 1 that extra try turns it into a plain failure.
• FIDO signing and TOTP on iOS.