fix(fs): cap the buffer allocated by the read command (#3626)

In plugins/fs/src/commands.rs read, `vec![0; len]` used the length sent
by the webview as is, so a huge `len` aborted the process on allocation
failure, and the whole buffer (not only the bytes read) was sent back.

The length is now capped to the rest of the file (at least 64 KiB) for
regular files and to 64 MiB otherwise, the allocation uses try_reserve so a
failure is an error, and the buffer is truncated to the bytes read. The JS
side only ever used the first `nread` bytes. Unit test for the cap.

.changes/fix-fs-read-allocation.md

@@ -0,0 +1,6 @@
+---
+fs: patch
+fs-js: patch
+---
+
+`FileHandle.read` no longer allocates a buffer of the requested size up front: the length is capped to the rest of the file (64 MiB for files of unknown size), an allocation failure is reported as an error instead of aborting the app, and only the bytes that were read are sent back to the webview.