A refused second session message makes the in-flight message lose its reply and stops the turn #452

Found while working #434.

Problem

A relay session message refused because a turn is already running (409, session: already processing a message) makes the first, in-flight relay session message lose its reply. The first CLI waits the full 5 minutes, exits 1 with session host answered 500 Internal Server Error (session: response timeout after 5m0s), and relay then stops the turn, although the agent answered long before.

Seen in the real app on the devbox (a host session: a long message, then a second message while it ran) and reproduced on main with an e2e test through the CLI: hold a local session's turn open, send a second message (409), release the turn; the first message never returns its reply.

Root cause

Manager.SendMessageSync registers its response collector in m.collectors[id] before SendMessage accepts the message, replacing the collector of the call already waiting. When SendMessage refuses the second call, its deferred cleanup deletes m.collectors[id], the first call's slot. The first call's collector then never sees turn_done, times out, and StopGeneration ends the turn.

Fix

A refused SendMessageSync leaves the in-flight call's collector in place: it does not replace a collector that is already waiting, and its cleanup removes only its own collector.

Acceptance criteria

• A relay session message refused while another message runs leaves the first one's reply intact.
• The first relay session message prints its reply when the turn ends, and exits 0.

Contract

• Owned files: internal/sessions/session/manager.go (SendMessageSync), e2e/features/bug452_refused_message_keeps_reply_test.go, docs/FEATURES.md (G1.05 names the test).