fix(client): survive handshake from a protocol-incompatible daemon

[georgeh0]

Fixes #237.

Summary

Upgrading 0.2.37 → 0.2.38 bricked every ccc command with ValidationError: Object missing required field 'pid'. Root cause: 0.2.38 (#214) added a required pid field to HandshakeResponse, so the reply of a still-running pre-upgrade daemon no longer decoded — the decode error fired before the client could see the ok=False version mismatch and restart the daemon. The same uncaught error also broke ccc daemon stop, i.e. the recovery path itself.

Three layers of fix:

• Wire compat: HandshakeResponse.pid gets a default (None), with a comment stating the rule — handshake fields added after a release must have defaults, since the handshake is the one message exchanged between mismatched client/daemon versions. With this alone, a stale 0.2.37 daemon's reply decodes, ok=False is seen, and the normal restart path fires.
• Decode hardening: an undecodable handshake reply now raises DaemonProtocolError instead of escaping as a raw msgspec error; _connect_and_handshake treats it like a version mismatch (restart on first contact, fail fast once a matching daemon was already ensured). This protects against any future wire drift, not just this field.
• Recovery path: stop_daemon tolerates the handshake failure and falls through to its SIGTERM/SIGKILL escalation, so ccc daemon stop always works against an incompatible daemon.

Workaround for users hitting this on 0.2.38 today: pkill -f cocoindex_code.daemon (or kill the PID from ~/.cocoindex_code/daemon.pid), then rerun any ccc command.

Test plan

• New test: a pre-0.2.38 handshake reply (no pid field) decodes (test_decode_handshake_response_from_pre_0_2_38_daemon)
• New test: undecodable handshake → daemon stopped and restarted, request served (test_connect_restarts_daemon_on_undecodable_handshake)
• New test: undecodable handshake after an ensured daemon fails fast without restart loop (test_connect_fails_fast_on_undecodable_handshake_after_ensured)
• New test: stop_daemon reaches the kill escalation instead of crashing (test_stop_daemon_escalates_past_undecodable_handshake)
• Full pre-commit suite green locally (ruff, ruff-format, mypy, full pytest)

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