fix(flow-chat): settle dialog turn completion unconditionally (light-tested)

[1688mengdie]

Closes #2656

Summary

handleDialogTurnComplete gates settlement on eventOwnsLatestSessionTurn, which requires the completed turn to be the last dialog turn; when an optimistic follow-up turn exists (or the machine context was reset by the failure path), the completion event is dropped and the session stays PROCESSING forever.

Ownership matching now prefers the machine's currentDialogTurnId, the failure path settles with a single FINISHING_SETTLED transition instead of ERROR_OCCURRED→RESET (which clears currentDialogTurnId and desynchronizes later ownership checks), and completion events settle unconditionally (BACKEND_STREAM_COMPLETED while PROCESSING plus an unconditional beginTurnCompletion). The ownsSessionSettlement mechanism stays in place at all other consumption points (:2703 failure cleanup, :2762 error unread, :2808 cancelled, :2871/:2929 interrupted, :2968).

Changes

• src/web-ui/src/flow_chat/services/flow-chat-manager/EventHandlerModule.ts: eventOwnsLatestSessionTurn prefers currentDialogTurnId; the failure path performs a single FINISHING_SETTLED transition; the two completion-path ownsSessionSettlement gates are removed so completion settles unconditionally.
• src/web-ui/src/flow_chat/services/flow-chat-manager/EventHandlerModule.test.ts: adds the regression case "settles the state machine when completion arrives for a non-latest turn".
• Commit: 6012aa4 — fix(flow-chat): settle dialog turn completion unconditionally

Testing

• pnpm vitest run src/flow_chat/services/flow-chat-manager/EventHandlerModule.test.ts → 1 test file passed, 48 tests passed, 0 failed (includes the new regression case)
• pnpm run type-check (tsc --noEmit) → exit 0, no errors

Environment

All platforms (frontend state machine); baseline 32f2427.

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AI-assisted change, reviewed and verified as described above (light-tested locally).