Fix AI video generation hanging forever when the provider reports failure

[giladresisi]

What kind of change does this PR introduce?

Bug fix.

Why was this change needed?

A production user approved an AI video generation in the agent chat and got: "..." spinner forever, their approval message gone after refresh, and 1 of their 30 monthly video credits burned.

Root cause: kie.ai runs content moderation asynchronously — /veo/generate returns 200 + taskId and the rejection only appears later through record-info. The veo3 poll loop's only exit condition was non-empty resultUrls: it never inspected successFlag, explicitly whitelisted top-level code: 400, had no attempt cap and no socket timeout. A failed task therefore polled forever. The never-settling promise meant useCredit's catch-based refund never ran (credit burned), the CopilotKit SSE stream never closed (eternal spinner), and Mastra never persisted the turn (that's the vanishing approval message).

Fix, keeping all provider-specific logic inside the veo3 provider:

• veo3.ts: bounded polling matching the HeyGen provider (180 × 10s ≈ 30 min, then throw). Keep polling only on successFlag: 0; any other flag throws with the provider's errorMessage (2/3 = failed; unknown values fail fast). Success reported without a URL throws. Top-level code !== 200 throws with the provider msg (safety wording then surfaces via generationError). AbortSignal.timeout(30000) on both fetches so a stalled socket can't hang. Once the callback rejects, the existing useCredit refund works untouched.
• generate.video.tool.ts: try/catch returning an { error } result (outputSchema fields made optional, same shape as upload.from.url.tool.ts) instead of throwing — the agent run completes, the stream closes cleanly, the turn is persisted.
• ai.video.tsx: the create-post modal no longer swallows errors — warning toaster on non-2xx, and on fetch rejection (network drop, where the backend may still finish and save to the media library).

Verified with a local mock kie.ai: baseline repro on the old code (failed task → endless polling with the credit row leaked unrefunded) and post-fix (code:400 / successFlag:2 / deadline paths all throw promptly with the credit refunded; chat replies with the failure message and the turn survives refresh; modal shows the toaster). Real happy-path generation verified end to end: 76s, video uploaded and saved to the media library, exactly one credit consumed.

Other information:

Follow-up commit: log the kie.ai taskId when a generation starts. While debugging the production report we could not correlate our logs with kie.ai's task history because the taskId was never recorded; with it logged, a failed generation can be looked up directly via kie.ai's record-info endpoint or dashboard.

Deliberately out of scope: out-of-app notification for timed-out generations, eager persistence of the user chat message before tool execution, error boundary around the copilot stream handler.

Checklist:

• I have read the CONTRIBUTING guide.
• I have signed the Contributor License Agreement (CLA) (ICLA for individuals, CCLA for entities).
• I confirm I have not used AI to submit this PR or generate code for it.
• I checked that there were no similar issues or PRs already open for this.
• This PR fixes just ONE issue

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