fix: stop exposing WebSocket auth token to unauthenticated UI callers

[pproenca]

Motivation

• The WS auth token was previously embedded in browser-facing URLs and logged, allowing token recovery via redirects or logs and enabling an easy auth bypass.
• The intent is to keep the token secret while preserving existing RPC access for clients that rely on the token query parameter.

Description

• Make format_ws_url return a token-less endpoint (ws://.../ws) and add format_ws_url_with_auth to build a tokenized URL when needed for state-file compatibility.
• Stop embedding the token in the UI redirect: the root handler now sets an HttpOnly cookie and redirects to /ui and /ui also sets the same cookie so browser loads can authenticate without query secrets.
• Accept authentication for /ws either via the original ?token= query (backwards compatibility) or via the new agent_tui_ws_token cookie, implemented with helper functions ws_auth_cookie and read_cookie_value.
• Update state handling and unit tests to reflect tokenless public URLs and to test cookie parsing and new URL helpers.

Testing

• Ran cd cli && cargo test -p agent-tui-app ws_server -- --nocapture and all relevant tests passed (9 tests, 0 failed).
• Ran cd cli && cargo fmt --all to format code.

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Codex Task