chore(studio): pin online scorer threads to the built-in preprocessor (#50981)

Pins the online scorers' `trace.getThread()` to the built-in `thread`
preprocessor, so we can set the Assistant project's default preprocessor
to a [custom one for
Topics](https://linear.app/supabase/issue/AI-1258/add-a-topics-preprocessor-that-caps-tool-results-in-assistant-traces)
without changing scorer input. `getThread()` otherwise [uses the project
default](https://github.com/braintrustdata/braintrust-sdk-javascript/blob/cc165a4843805b531645ddb1d27969204aab9ade/js/src/trace.ts#L807).

Ref AI-1258


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Corrected thread retrieval to use Braintrust’s thread preprocessor,
ensuring evaluation traces are processed consistently.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

apps/studio/evals/scorer.test.ts

@@ -70,7 +70,7 @@ describe('scorers with online (null) output', () => {
     const { trace, getThread } = mockTrace(THREAD_WITH_DOCS_URL)
     const result = await runUrlValidityScorer(null, trace)
 
-    expect(getThread).toHaveBeenCalled()
+    expect(getThread).toHaveBeenCalledWith({ preprocessor: 'thread' })
     expect(result).toMatchObject({ name: 'URL Validity', score: 1, metadata: { urls: [DOCS_URL] } })
 
     vi.unstubAllGlobals()

apps/studio/evals/trace-utils.ts

@@ -223,5 +223,6 @@ export function getThreadPartsFromThread(thread: unknown[]): Transcript {
  * in a way the offline path isn't).
  */
 export async function getThreadParts(trace: Trace): Promise<Transcript> {
-  return getThreadPartsFromThread(await trace.getThread())
+  // Pinned to not rely on project's default preprocessor, which may be custom for Topics
+  return getThreadPartsFromThread(await trace.getThread({ preprocessor: 'thread' }))
 }