db_bench and db_stress support for WriteBufferManager flush policy

[rban1]

Summary:
Exposes the WriteBufferFlushPolicy added in D112396106 to the benchmarking and stress-testing tools, per the repo convention that a new option ships with db_bench and stress coverage.

db_bench: new -write_buffer_manager_flush_policy flag accepting oldest, largest, and largest_across_dbs, parsed by a StringToWriteBufferFlushPolicy helper that mirrors the existing StringToAdmissionPolicy idiom. Also adds -write_buffer_manager_allow_stall, since the WriteBufferManager constructor takes allow_stall immediately before the policy and db_bench previously had no way to set it. Both are consumed at the single WriteBufferManager construction site in InitializeOptionsFromFlags. Note the flags only take effect when -db_write_buffer_size or -cost_write_buffer_to_cache is set, since otherwise db_bench creates no WriteBufferManager; -num_multi_db=N already shares one manager across all DBs, which is what largest_across_dbs needs.

db_stress: new -wbm_flush_policy int flag (0 = oldest, 1 = largest, 2 = largest across DBs), following the prepopulate_block_cache / compaction_style int-enum idiom rather than adding a string parser. It is passed at the shared wbm construction site in db_stress_tool.cc.

db_crashtest.py: randomizes wbm_flush_policy over [0, 1, 2], and finalize_and_sanitize forces it back to 0 when use_write_buffer_manager is disabled, so repro commands do not carry a policy that has no effect.

No behavior change to the engine itself; this diff is tooling only.

Differential Revision: D114638570

[meta-codesync]

@rban1 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D114638570.

[github-actions]

⚠️ clang-tidy: 3 warning(s) on changed lines

Completed in 335.5s.

Summary by check

Check	Count
modernize-make-shared	3
Total	3

Details

db/db_write_buffer_manager_test.cc (2 warning(s))

db/db_write_buffer_manager_test.cc:862:32: warning: use std::make_shared instead [modernize-make-shared]
db/db_write_buffer_manager_test.cc:948:32: warning: use std::make_shared instead [modernize-make-shared]

tools/db_bench_tool.cc (1 warning(s))

tools/db_bench_tool.cc:5201:36: warning: use std::make_shared instead [modernize-make-shared]