…mitted rowsets (#69980)

### What problem does this PR solve?

Issue Number: None

Related PR: #64995

Problem Summary:

A cloud BE logs a burst of WARN lines with a full stack trace like

meet error status: [NOT_FOUND]File not found in global packed index:
data/<tablet>/<rowset>_0.idx

while compaction reads its input rowsets. In one instance a 4-hour BE
log had 42k
such lines over 21k paths in 158 tablets, yet every compaction
succeeded.

Reproduce: write a rowset whose segment file is smaller than
small_file_threshold_bytes (so it is packed) but whose inverted index
file is
larger (so it is uploaded as a regular object), then compact or query
it. Each
open of the regular .idx file prints the WARN with a stack, twice per
compaction
(eligibility check and the merge itself).

Root cause: #64995 made PackedFileSystem::open_file_impl fall back to
PackedFileManager::get_packed_slice_location when a path is not in
_index_map,
so that the file cache preload can read back a segment of the rowset
being
written, whose rowset meta does not exist yet. The fallback was added
unconditionally, so it also runs for a PackedFileSystem built from a
rowset
meta. That index map already holds every packed slice of the rowset, so
a miss
means a regular file, and the probe can only fail. The failure is a
Status::NotFound, and NOT_FOUND records a stack trace and logs a WARN as
soon as
it is constructed, before the caller falls back to opening the regular
file.

Fix:
- Only consult the manager when the index map is not initialized, i.e.
on the
write path, the same split exists_impl and file_size_impl already make.
- Return NOT_FOUND without a stack trace from get_packed_slice_location:
on the
  write path a miss is also expected, for files too large to be packed.

After the fix, reading committed rowsets no longer takes the global
index lock
or logs anything for regular files; the write-path preload still finds
packed
slices through the manager.

### Release note

Fix noisy "File not found in global packed index" WARN logs with stack
traces
when reading rowsets that mix packed and regular files in cloud mode.