filer: support nanosecond timestamps for conditional delete (#11339) (#12071)

* filer: support nanosecond timestamps for conditional delete (#11339)

* shell: fs.verify prune carries the entry mtime in nanoseconds

IfNotModifiedAfter now resolves to sub-second precision on the filer,
so pass the looked-up mtime_ns along with mtime; the re-lookup check
catches same-second rewrites that share the second, and the test mock
mirrors the filer's seconds/nanoseconds cutoff.

* shell: volume.fsck guards empty-directory deletes with mtime nanoseconds

The looked-up mtime_ns joins the seconds in IfNotModifiedAfter so a
directory rewritten within the same second is not dropped; the stub
filer mirrors the filer's scale heuristic and the race fixtures use a
realistic mtime so the nanosecond branch engages.

* filer: cover the seconds guard and equality boundary

Test second-resolution cutoffs on both sides of the entry's second and
the exact-equality nanosecond guard that still deletes.

* filer: compare the nanosecond guard as a time, not UnixNano

UnixNano overflows for mtimes outside 1678-2262, letting a far-future
entry slip past the guard; time.After compares exactly at every epoch.
The seconds branch keeps Unix() so a same-second cutoff still deletes.

* filer_pb: add explicit if_not_modified_after_ns to DeleteEntryRequest

* filer: prefer explicit nanoseconds guard, keep if_not_modified_after as seconds

* shell: send seconds and nanoseconds delete cutoffs, skip unexpressible mtimes

* filer: merge conditional-delete guards into one nanosecond cutoff

DeleteEntryMetaAndData carried both the seconds and nanoseconds guards
even though the precedence between them is fixed, so every caller passed
a dead parameter. The gRPC handler now folds if_not_modified_after into
nanoseconds at the boundary, where (sec+1)*1e9-1 keeps the whole-second
cutoff exact, and the filer takes a single ifNotModifiedAfterNs.

* filer: hold the conditional-delete cutoff as a time, not int64 nanoseconds

A seconds-only guard at or past year 2262 saturated to MaxInt64
nanoseconds (~2262), so a delete whose cutoff matched a far-future mtime
was skipped while the request still reported success, and any entry
between 2262 and the real cutoff survived pruning. The merged cutoff is
now a time.Time, where a whole-second guard maps to the last nanosecond
of its second for any int64 second.

---------

Co-authored-by: user.email <chrislusf@users.noreply.github.com>
Co-authored-by: Chris Lu <chris.lu@gmail.com>

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

@@ -524,8 +524,9 @@ message DeleteEntryRequest {
     bool ignore_recursive_error = 6;
     bool is_from_other_cluster = 7;
     repeated int32 signatures = 8;
-    int64 if_not_modified_after = 9;
+    int64 if_not_modified_after = 9;    // unix time in seconds
     bool keep_remote_object = 10;
+    int64 if_not_modified_after_ns = 11; // unix time in nanoseconds; overrides if_not_modified_after when set
 }
 
 message DeleteEntryResponse {