fix(app): treat benign 404 as recovery in processInProgressConversation (#9241)

[kodjima33]

What

processInProgressConversation() in app/lib/backend/http/api/conversations.dart used to crash-report every non-200 response from POST /v1/conversations. A benign 404 now short-circuits to a debug log instead of a crash report; all other non-200 statuses still report a crash.

Why

The backend endpoint (backend/routers/conversations.py:162-165) raises 404 "Conversation in progress not found" when there is no in-progress conversation to process:

conversation = retrieve_in_progress_conversation(uid)
if not conversation:
    raise HTTPException(status_code=404, detail="Conversation in progress not found")
redis_db.remove_in_progress_conversation_id(uid)

This is exactly the race described in the issue:

Flutter crashes/reports when processInProgressConversation gets a benign 404 due to race with WS auto-finalize.

When the WS auto-finalize path finalizes the in-progress conversation and clears its Redis pointer first, a client-initiated create that races in finds nothing to process → 404. The conversation is already finalized, so there is nothing to recover — but the old code reported it as Exception('Failed to create conversation'), flooding crash reporting with noise that has no user-visible impact. The caller in capture_controller.dart already recovers gracefully on a null return (it removes the '0' processing placeholder).

Change

• 404 → Logger.debug(...) and return null (benign, already-finalized).
• Every other non-200 → unchanged crash report.
• Extracted isBenignInProgressConversationCreateStatus(int) so the crash-vs-skip decision is covered by a hermetic regression test (the makeApiCall / crashReporter singletons aren't injectable — this mirrors the existing sync_response_handling_test.dart pattern of testing branching logic via a minimal abstraction).

Verified

• Traced the exact backend 404 source and confirmed the caller already handles null without side effects.
• Added app/test/unit/process_in_progress_conversation_test.dart asserting 404 is benign and 304/400/401/409/429/5xx still report.
• Could not run app/test.sh in this environment — the build machine has no Flutter/Dart SDK. CI (app tests) will exercise the new unit test. The change is a single status-code branch with no visual surface.

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