Disable open_read_only_one_in when mmap_read is enabled (#15011)

Summary:
Pull Request resolved: https://github.com/facebook/rocksdb/pull/15011

Root cause: `open_read_only_one_in` can create a read-only DB instance on the same live directory as the primary DB. When the stress run also enables `mmap_read=1`, that read-only instance can hold mmap-backed slices into SST files while the primary continues compacting and deleting obsolete files. If `SstFileManager` uses chunked truncation (`sst_file_manager_bytes_per_truncate > 0`), Linux can raise SIGBUS when the read-only path later touches a page whose backing SST was truncated or removed. The range tombstone fragmenter/comparator is where the invalid mmap-backed key slice was touched, not necessarily the root cause.

Evidence from T281873950 / Sandcastle job `63050395314610170`: the crashing command had `--mmap_read=1`, `--open_read_only_one_in=16`, and `--sst_file_manager_bytes_per_truncate=1048576`. The crash stack was `BlockBasedTable::ReadRangeDelBlock()` -> `FragmentedRangeTombstoneList::FragmentTombstones()` -> `std::set<ParsedInternalKey>::emplace()` -> comparator -> `memcmp`, ending in signal 7/SIGBUS. The crash DB LOG shows `allow_mmap_reads: 1`; table `104662.sst` was created cleanly at size `6257069` with range tombstones and was later deleted by the primary. The artifact does not show persistent SST corruption or checksum failure, and injected read faults would normally surface as RocksDB statuses rather than an OS SIGBUS.

Why this started surfacing: D111995911 / `rFBS0f7a2ef93e63cfa6677b6fa32c1bd34eb818c978` added randomized read-only-on-primary coverage via `open_read_only_one_in` on 2026-07-20. Before that change, blackbox crash test did not randomly keep a separate read-only DB open on the primary live directory, so the existing `mmap_read + SstFileManager truncation` hazard lacked the second reader needed to trigger SIGBUS. The failure is therefore a probabilistic exposure of the new stress mode when it combines with mmap reads and chunked SST truncation.

Fix: disable `open_read_only_one_in` in the existing `mmap_read=1` sanitizer path. This preserves read-only-on-primary stress coverage for non-mmap runs while avoiding an OS-level mmap/truncation hazard that RocksDB cannot convert into a normal `Status`.

Reviewed By: jaykorean

Differential Revision: D113774892

fbshipit-source-id: f82ec2924c12f4512aeeb7a913659dc9a72a4c2f

tools/db_crashtest.py

@@ -944,6 +944,7 @@ def finalize_and_sanitize(src_params):
         dest_params["use_direct_reads"] = 0
         dest_params["use_direct_io_for_compaction_reads"] = 0
         dest_params["multiscan_use_async_io"] = 0
+        dest_params["open_read_only_one_in"] = 0
     if dest_params.get("min_tombstones_for_range_conversion", 0) > 0:
         # SQFC range-query filtering installs ReadOptions::table_filter on
         # iterators. Read-write iterators reject table_filter when read-path