Add multi-DB stress testing support

[hx235]

Summary:
Add --num_dbs flag for running N independent StressTest instances sharing
one Env. Each DB gets its own paths, fault injection, and stress threads.

Motivation: validate shared-resource behavior (block cache,
WriteBufferManager, RateLimiter, compressed secondary cache) across
concurrent DB instances, and provide the multi-DB harness for future
shared components like the auto-tuner.

Note: multi-DB mode (--num_dbs > 1) is not yet enabled in CI. The
db_crashtest.py randomization is commented out with a TODO at line 481.
Enabling it in CI will be done in a follow-up change.

Per-DB path layout:
--db=/tmp/mydb → /tmp/mydb/db_0, /tmp/mydb/db_1, ...
--expected_values_dir=/tmp/ev → /tmp/ev/db_0, /tmp/ev/db_1, ...
--secondaries_base=/tmp/sec → /tmp/sec/db_0, /tmp/sec/db_1, ...

Per-DB isolation:

• Per-DB FaultInjectionTestFS (independent fault injection per DB)
• Per-DB crash callback (async-signal-safe C array of raw pointers)
• Per-DB RunStressTestImpl() with own SharedState — all features
  (backup, checkpoint, secondary DB, continuous verification, remote
  compaction) work correctly in multi-DB mode via per-DB paths

Shared resources (across all DBs):

• Env and thread pools (compaction, flush, bottom-pri)
• Block cache
• WriteBufferManager
• RateLimiter

Guards (features disabled/rejected in multi-DB mode):

• clear_column_family_one_in: pre-existing CF handle race condition
• compressed_secondary_cache: per-DB CompressedCacheSetCapacityThread
  instances race on the shared global cache (SetCapacity(0)/SetCapacity(size))
• test_multi_ops_txns: uses a single global key_spaces_path file that
  persists key range descriptors — multiple DBs would overwrite each
  other's ranges, causing layout corruption on reopen

Python driver (db_crashtest.py) changes:

• cleanup_after_success() forwards num_dbs, expected_values_dir,
  secondaries_base, test_secondary, and continuous_verification_interval
  to --destroy_db_and_exit so all directories are properly cleaned
• Lambda params (expected_values_dir, num_dbs, test_secondary)
  materialized early in blackbox/whitebox because cleanup_after_success
  reads them from cmd_params which stores raw lambdas from default_params

Bonus cleanup in --destroy_db_and_exit:

• Cleans expected_values_dir via Env::Default() (always local, even
  when the DB itself lives on a remote env_uri/fs_uri)
• Cleans secondaries_base including auto-generated default path when
  test_secondary or continuous_verification_interval is enabled
  These were pre-existing leaks in single-DB mode too, but fixed here
  because the Python forwarding above requires the C++ side to handle
  these flags.

Test Plan:
make db_stress
Result: PASSED

Single-DB backward compatibility (verification successful):
./db_stress --max_key=10000 --ops_per_thread=5000 --destroy_db_initially --db=/tmp/rocksdb_v_single
Result: PASSED

Multi-DB basic, 3 DBs (all verified):
./db_stress --num_dbs=3 --max_key=10000 --ops_per_thread=5000 --destroy_db_initially --db=/tmp/rocksdb_v_multi
Result: PASSED

Multi-DB + explicit expected_values_dir (all verified):
./db_stress --num_dbs=3 --max_key=10000 --ops_per_thread=5000 --destroy_db_initially --expected_values_dir=/tmp/test_ev --db=/tmp/rocksdb_v_evdir
Result: PASSED

[ongoing] CI stress test large-scale rehearsal

[meta-codesync]

@hx235 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D96575551.

[github-actions]

⚠️ clang-tidy: 10 warning(s) on changed lines

Completed in 150.6s.

Summary by check

Check	Count
concurrency-mt-unsafe	6
cppcoreguidelines-avoid-non-const-global-variables	4
Total	10

Details

db_stress_tool/db_stress_common.cc (2 warning(s))

db_stress_tool/db_stress_common.cc:25:56: warning: variable 'shared_wbm' is non-const and globally accessible, consider making it const [cppcoreguidelines-avoid-non-const-global-variables]
db_stress_tool/db_stress_common.cc:26:49: warning: variable 'shared_rate_limiter' is non-const and globally accessible, consider making it const [cppcoreguidelines-avoid-non-const-global-variables]

db_stress_tool/db_stress_tool.cc (8 warning(s))

db_stress_tool/db_stress_tool.cc:47:5: warning: variable 'g_fault_fs_for_crash' is non-const and globally accessible, consider making it const [cppcoreguidelines-avoid-non-const-global-variables]
db_stress_tool/db_stress_tool.cc:48:12: warning: variable 'g_num_dbs_for_crash' is non-const and globally accessible, consider making it const [cppcoreguidelines-avoid-non-const-global-variables]
db_stress_tool/db_stress_tool.cc:101:7: warning: function is not thread safe [concurrency-mt-unsafe]
db_stress_tool/db_stress_tool.cc:465:5: warning: function is not thread safe [concurrency-mt-unsafe]
db_stress_tool/db_stress_tool.cc:470:5: warning: function is not thread safe [concurrency-mt-unsafe]
db_stress_tool/db_stress_tool.cc:495:7: warning: function is not thread safe [concurrency-mt-unsafe]
db_stress_tool/db_stress_tool.cc:548:29: warning: function is not thread safe [concurrency-mt-unsafe]
db_stress_tool/db_stress_tool.cc:567:11: warning: function is not thread safe [concurrency-mt-unsafe]

[meta-codesync]

@hx235 has imported this pull request. If you are a Meta employee, you can view this in D96575551.

[meta-codesync]

@hx235 has imported this pull request. If you are a Meta employee, you can view this in D96575551.

[meta-codesync]

@hx235 has imported this pull request. If you are a Meta employee, you can view this in D96575551.