generateStream: no empty-response guard, no fallback, and a raw provider error yielded as content

Split out of #231, which carries this as a bullet rather than a tracked item. #223's review said "tracking with #228" — that reference is wrong: #228 is the QA chat SSE cross-chunk buffer bug in apps/web/src/hooks/use-qa-chat.ts, a different subject. So this has had no issue of its own.

generate() was fixed by #223 (merged as 825d0981): it now reads every text block and throws 'Model response contained no usable text' on none, caught by the never-crash fallback. generateStream has the same two defects, both unfixed.

1. No empty-response guard, and no fallback at all

packages/outpost/ai/src/generator.ts:196-206 yields only content_block_delta/text_delta events. A completion with no text yields nothing — the consumer sees a successful empty stream. There is no degraded signal on this path and no apology fallback, so it is strictly worse than the pre-#223 non-streaming behaviour, which at least returned an empty string a caller could test.

2. The catch yields a raw provider error as ordinary content

:207-208:

} catch (error) {
    yield `\n\n_Error generating response: ${error instanceof Error ? error.message : String(error)}_`;
}

That is provider error text placed into the response body. pipeline.generateStreamingResponse buffers what it yields, so a 500 becomes publishable content. Trap rather than live bug — no production caller reaches it today, which is also why it has stayed unnoticed. Overlaps #146 (the streaming path still reads the pre-#143 groundedness signal) and should probably be fixed in the same pass.

Why it hasn't bitten yet

generateStream's only caller is pipeline.generateStreamingResponse, which nothing in the queue invokes. The moment streaming is wired to a real channel, both defects are live.

Test gaps

No coverage for an empty stream and none for the catch path. Both are straightforward with the aimock recipe #223 used — the empty case needs a mocked stream that yields no text delta.

Start in packages/outpost/ai/src/generator.ts:196-208 and compare generateStream with the generate behavior fixed by #223. Trace how pipeline.generateStreamingResponse consumes the yielded values, then use the #223 aimock recipe to cover an empty stream and the catch path. Done means empty responses receive the intended fallback behavior and provider errors are not emitted as ordinary content.