refactor(sse): stop isClaudeEventPayload claiming a narrowing it never performs (#8557)

Eight TS2339s in stream.ts read properties off `never`. `never` here did not
mean unreachable code — it meant a type predicate was lying.

  function isClaudeEventPayload(payload: unknown): payload is JsonRecord
  ...
  const flushedParsed = bufferedPayload as JsonRecord;
  const isClaude = isClaudeEventPayload(flushedParsed);
  } else if (!isClaude) {   // JsonRecord minus JsonRecord = never

The predicate answers "does this payload carry a Claude event type?" — a
question about contents, not type. A Claude event and a non-Claude one are both
JsonRecord, so `payload is JsonRecord` narrows nothing; applied to an argument
already typed JsonRecord it collapsed the negative branch to `never`, taking
`.id` and `.choices` with it.

Changed the return type to `boolean`. No call site depended on the narrowing:
both other callers pass the value to updateClaudeEmptyResponseLifecycle(), whose
parameter is `unknown`, and passthroughTailProcessor.ts already declares this
function as `(payload: unknown) => boolean` — so this aligns the definition with
how the codebase already consumes it.

280 -> 272, zero new, on a line-number-agnostic diff of the full tsc error set.

The diff is one line, and a type predicate has no runtime representation, so the
emitted JavaScript is unchanged.

No test added, and no gap to fill: the `never`-typed branch is real, reachable
and already covered from both sides — stream-numeric-ids.test.ts exercises
"normalizes numeric id in final chunk without trailing newline" (the flush path
with a non-Claude payload, i.e. the branch TS believed impossible) and "Claude
passthrough does not normalize numeric ids" (the other arm). 338/338 across the
48 SSE/passthrough suites.

No explanatory comment either: stream.ts sits exactly at its frozen file-size
cap (2887), so a single added line fails check:file-size. The reasoning lives
here and in the PR rather than costing a baseline bump on a frozen file.

Co-authored-by: backryun <busan011@ormbiz.co.kr>

open-sse/utils/stream.ts

@@ -485,7 +485,7 @@ function getClaudeEventType(payload: unknown): string | null {
   return typeof type === "string" ? type : null;
 }
 
-function isClaudeEventPayload(payload: unknown): payload is JsonRecord {
+function isClaudeEventPayload(payload: unknown): boolean {
   return getClaudeEventType(payload) !== null;
 }