fix(storage): propagate VikingFS.mkdir backend errors instead of swallowing them

[hobostay]

Problem

VikingFS.mkdir() (openviking/storage/viking_fs.py) catches every exception from the backend mkdir call, but the except block has no re-raise:

try:
    await self._async_agfs.mkdir(path, fs_ctx=self._pathlock_fs_ctx(ctx, lease_ref))
except Exception as exc:
    message = str(exc).lower()
    already_exists = "exist" in message or "already" in message
    if exist_ok and already_exists:
        return
    # <- anything else falls through and is silently discarded

Any failure that is not an already-exists error tolerated by exist_ok=True — permission denied, quota/resource-exhausted, I/O errors, lock-lease violations, and even already-exists errors with the default exist_ok=False — is silently swallowed, and mkdir() returns as if the directory had been created.

mkdir is on the write hot path (ovpack import, parsers, session, privacy service, and mv's internal copy). Callers reasonably assume a returned mkdir means the directory exists; downstream writes then fail later with confusing secondary errors, and the original, actionable error is lost.

Fix

Re-raise the original exception unless it is an already-exists error tolerated by exist_ok=True:

            if exist_ok and already_exists:
                return
            raise

One-line change; no behavior change for the already-exists + exist_ok=True path.

Tests

tests/misc/test_mkdir.py was stale: it mocked fs.agfs.mkdir, but mkdir() goes through the AsyncAGFSClient wrapper (self._async_agfs). Four of its five tests failed on main, and the fifth passed only because the swallowed AttributeError from the missing mock made mkdir() return early — the very bug this PR fixes.

• Updated the existing tests to mock _async_agfs.mkdir and to match the current create-then-tolerate-exists semantics.
• Added regression tests: backend errors propagate with both exist_ok=False and exist_ok=True (non-exists errors), and exist_ok=False surfaces already-exists errors.

tests/misc/test_mkdir.py: 8 passed

Full tests/storage, tests/misc, and tests/service runs show no new failures versus main (the only delta is the 4 previously-failing stale mkdir tests now passing).