test(ai): cover the partialOutput PRESERVE branch of the ai@6.0.134 patch (#184, review F1)

The patch forks createOutputTransformStream: output==null skips partialOutput
(the OOM fix, already tested), output!=null preserves the original cumulative
accumulation. Only the skip branch was tested; the preserve branch — on which the
patch's "byte-identical when an output strategy is set" safety claim rests — had no
coverage, so a future re-port (patches are re-created via `pnpm patch` on every ai
bump) could silently route output-set calls into the skip branch and leave
partialOutput empty for object/text-output consumers, uncaught.

Add a 4th test: streamText({ ..., experimental_output: Output.text() }), drain
textStream, collect experimental_partialOutputStream, and assert it is non-empty and
cumulative (last partial == full text "Hello, world!"). Reuses the existing
makeModel() harness. Verified on the patched dist: partials are
["Hello","Hello, ","Hello, world!"]. `npx jest ai-sdk-partial-output.patch.spec.ts`
→ 4 passed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/integrations/ai/ai-sdk-partial-output.patch.spec.ts

@@ -1,5 +1,5 @@
 import { readFileSync } from 'fs';
-import { streamText } from 'ai';
+import { streamText, Output } from 'ai';
 import { MockLanguageModelV3, simulateReadableStream } from 'ai/test';
 
 /**
@@ -80,6 +80,38 @@ describe('ai@6.0.134 pnpm patch: no partialOutput accumulation without an output
     expect(partials).toEqual([]);
   });
 
+  it('preserves cumulative partialOutput when the caller DOES request an output strategy', async () => {
+    // PRESERVE-BRANCH GUARD: the patch only short-circuits partialOutput when
+    // `output == null`. When an output strategy IS set (here Output.text()),
+    // createOutputTransformStream must fall through to the ORIGINAL code path
+    // and keep publishing cumulative snapshots, so object/text-output consumers
+    // behave byte-identically to unpatched ai. A careless re-port that routed
+    // output-set calls into the skip branch would leave partialOutput empty and
+    // silently break those consumers — this test is the tripwire for that.
+    const result = streamText({
+      model: makeModel(),
+      prompt: 'hi',
+      experimental_output: Output.text(),
+    });
+
+    // Drain the primary stream fully and accumulate the complete output text.
+    let fullText = '';
+    for await (const delta of result.textStream) {
+      fullText += delta;
+    }
+
+    const partials: string[] = [];
+    for await (const partial of result.experimental_partialOutputStream) {
+      partials.push(partial);
+    }
+
+    // With a strategy set, partialOutput must be PRESERVED (non-empty) and
+    // cumulative: the last emitted partial equals the full accumulated text.
+    expect(partials.length).toBeGreaterThan(0);
+    expect(partials[partials.length - 1]).toBe(fullText);
+    expect(fullText).toBe('Hello, world!');
+  });
+
   it('both installed dist builds (CJS and ESM) carry the patch marker', () => {
     // Secondary guard: pins the patch to BOTH bundles the SDK ships, since
     // the NestJS server consumes CJS while other tooling may load ESM.