fix(cache): atomic file writes and size-validated cache reads

A failed/interrupted GitHub fetch could leave a 0-byte or truncated
file in the local cache. Subsequent reads happily streamed the empty
content as the file's body — visible to users as an "Empty file" with
HTTP 200. Reproduced on artifact-70B6/Lethe/configs.py (#694).

- FileSystem.write: stream into a sibling .tmp and rename into place
  only on finish. Stream errors discard the tmp and leave any prior
  cached file untouched. Drop the utf-8 encoding that was silently
  corrupting binary blobs.
- GitHubStream.getFileContentCache: accept an expected size and treat
  cached.size < expected as a poisoned cache (truncated fetch) → rm
  and re-fetch. cached.size >= expected is accepted, which keeps
  Git LFS-resolved files (whose FileModel.size is the pointer size)
  working.
- AnonymizedFile: expose size() and pass it through to the streamer
  alongside sha so the cache check has the upstream size.

Existing poisoned entries self-heal on next access.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/streamer/route.ts

@@ -42,6 +42,8 @@ router.post("/", async (req: express.Request, res: express.Response) => {
   const repoFullName = req.body.repoFullName.split("/");
   const repoId = req.body.repoId;
   const fileSha = req.body.sha;
+  const fileSize: number | undefined =
+    typeof req.body.size === "number" ? req.body.size : undefined;
   const commit = req.body.commit;
   const filePath = req.body.filePath;
   const anonymizerOptions = req.body.anonymizerOptions;
@@ -58,7 +60,7 @@ router.post("/", async (req: express.Request, res: express.Response) => {
     const content = await source.getFileContentCache(
       filePath,
       repoId,
-      () => fileSha
+      () => ({ sha: fileSha, size: fileSize })
     );
     const mime = lookup(filePath);
     if (mime && !filePath.endsWith(".ts")) {