fix(cache): key the object body cache on data_dir for write-uniqueness (#4703)

* refactor(object-data-cache): derive Default for ObjectDataCacheGetRequest

The GET request literal is hand-listed field-by-field across ~13 test sites in
two crates. Adding `mod_time_unix_nanos` in backlog#1111 had to touch every one
and still missed a literal, producing a compile error caught only in a later
CI lane. Derive `Default` and spread the engine-crate literals so the next
field addition is absorbed rather than fanned out.

Co-Authored-By: heihutu <heihutu@gmail.com>

* fix(cache): key the object body cache on data_dir for write-uniqueness

The cache key's correctness rested on being write-unique, but its only
write-scoped component was `mod_time` — a wall-clock timestamp that is not
monotonic and can be absent. Two writes that collide on MD5 (same etag+size)
and land on an equal mod_time (clock skew, same-tick, or absent) derived the
same key, so a node that never saw the overwrite could serve the previous body
for up to the TTL, and the same collision turned the fill-after-invalidation
race into a serving bug. This is the store's strong-read-after-write guarantee
leaning on a probabilistic argument.

Add `data_dir` — the xl.meta directory UUID ecstore regenerates on every body
write — as the primary write-unique anchor:
- surface `data_dir: Option<Uuid>` on ObjectInfo, copied from FileInfo in the
  single GET-path constructor `from_file_info`;
- carry it into ObjectDataCacheKey as `data_dir_u128` (held as u128 to keep the
  engine crate free of a uuid dependency), derived in the one planner site both
  the ecstore hook and the usecase layer share, so both produce an identical
  key by construction;
- keep `mod_time` as a second anchor and `etag+size` as belt-and-braces; an
  absent data_dir falls back to the prior behavior — strict improvement, no
  regression.

Two writes distinct only by data_dir now derive different keys even under an
MD5 collision with identical mod_time — the case mod_time alone cannot cover.

Blast radius is compiler-guarded: ObjectInfo derives Default and every real
construction site uses `..Default::default()`, so only from_file_info and one
full-literal test needed the field. The three P0 body_cache_hook_e2e
regressions, engine (80), and app (36) suites pass unchanged; a mutation that
severs the planner wiring fails planner_key_changes_with_data_dir.

Refs: backlog#1111, backlog#1118

Co-Authored-By: heihutu <heihutu@gmail.com>

* fix(cache): thread data_dir field through the merged mutation-hook test

Merging main (which landed the object-mutation-hook work, backlog#1131) brought
in a GetRequest test literal that predates the data_dir field. Spread it via
`..Default::default()` — the derive(Default) added here means this is the last
such hand-listed literal to need touching.

Co-Authored-By: heihutu <heihutu@gmail.com>

---------

Co-authored-by: heihutu <heihutu@gmail.com>

crates/ecstore/src/object_api/types.rs

@@ -172,6 +172,10 @@ pub struct ObjectInfo {
     pub parity_blocks: usize,
     pub data_blocks: usize,
     pub version_id: Option<Uuid>,
+    /// xl.meta directory UUID for this version, regenerated on every body write.
+    /// A write-unique token: the object data cache keys on it so an overwrite
+    /// cannot be served the previous body under an MD5 collision (backlog#1111).
+    pub data_dir: Option<Uuid>,
     pub delete_marker: bool,
     pub transitioned_object: TransitionedObject,
     pub restore_ongoing: bool,
@@ -211,6 +215,7 @@ impl Clone for ObjectInfo {
             parity_blocks: self.parity_blocks,
             data_blocks: self.data_blocks,
             version_id: self.version_id,
+            data_dir: self.data_dir,
             delete_marker: self.delete_marker,
             transitioned_object: self.transitioned_object.clone(),
             restore_ongoing: self.restore_ongoing,
@@ -515,6 +520,7 @@ impl ObjectInfo {
             parity_blocks: fi.erasure.parity_blocks,
             data_blocks: fi.erasure.data_blocks,
             version_id,
+            data_dir: fi.data_dir,
             delete_marker: fi.deleted,
             mod_time: fi.mod_time,
             size: fi.size,