filer: let PATCH_EXTENDED replace Entry.content (#9654)

* filer: let PATCH_EXTENDED replace Entry.content

PATCH_EXTENDED merges extended attributes under the per-path lock, reading the
entry fresh, so concurrent patches to different keys don't clobber each other.
Some single-key state lives in Entry.content rather than an extended attribute
(e.g. the S3 bucket metadata blob). Add set_content/content to the mutation so a
patch can replace content the same way -- read fresh, set content, preserve the
rest -- letting a content write and an extended-attribute write on the same
entry serialize on the lock instead of racing whole-entry rewrites.

* Update weed/server/filer_grpc_server.go

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

* filer: test set_content FileSize sync; note chosen content-patch approach

Cover the FileSize behavior of a set_content patch: a file's size follows the
new content length (including when it shrinks), a directory's stays zero. Also
document, in the bucket-config design, that extending PATCH_EXTENDED with
set_content is the implemented path for content-backed config.

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

other/java/client/src/main/proto/filer.proto

@@ -312,6 +312,8 @@ message ObjectMutation {
     bool is_delete_data = 7;               // DELETE: also delete chunk data
     bool is_recursive = 8;                 // DELETE: recurse into a directory
     Recompute recompute = 9;               // RECOMPUTE_LATEST parameters
+    bool set_content = 10;                 // PATCH_EXTENDED: replace Entry.content with content
+    bytes content = 11;                    // PATCH_EXTENDED: new Entry.content when set_content
 }
 
 // Recompute re-derives a pointer entry (directory/name on the mutation) from the