[Bug]: chat UI can stay stuck processing when a dialog turn completion event is dropped

## 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.

## Area

src/web-ui/src/flow_chat/services/flow-chat-manager/EventHandlerModule.ts (+ its test file).

## Reproduction or evidence

At 32f2427697, eventOwnsLatestSessionTurn (:183-192) still prioritizes dialogTurns.at(-1); handleDialogTurnComplete :2648/:2658 still gate both transitions on ownsSessionSettlement; the failed path (:2748-2749) still performs the ERROR_OCCURRED→RESET double transition. A turn completed while a newer optimistic turn exists leaves the state machine in PROCESSING indefinitely. Regression test: settles the state machine when completion arrives for a non-latest turn.

## Environment

All platforms (frontend state machine); baseline 32f2427697.

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AI-assisted change. Testing: verified locally (`pnpm vitest run EventHandlerModule.test.ts` 48 passed, 0 failed, including the new regression case; `pnpm run type-check` (tsc --noEmit) exit 0 with no errors).

Start with src/web-ui/src/flow_chat/services/flow-chat-manager/EventHandlerModule.ts, especially eventOwnsLatestSessionTurn and handleDialogTurnComplete, then read its test file. Reproduce the dropped completion case and run pnpm vitest run EventHandlerModule.test.ts plus pnpm run type-check; done means the non-latest completion settles the state machine without breaking existing tests.

Written by the indexing model from the issue text.