test(persist): Stabilize flaky session write fixture on nightly CI

[mhiro2]

Summary

• Stabilize the persist_sessions_spec write fixture that intermittently failed with Store write failed on the nightly CI job.
• Switch the write_and_wait helper from the async store.write (libuv fs_open/fs_write/fs_rename callback chain) to the synchronous store.write_sync, making setup/teardown writes deterministic and eliminating the transient async-fs race.
• Coverage of the async write path is preserved: it is still exercised by the save_current (async) + wait_for_store tests and the "async save write fails" test.
• Verified locally by running the spec 10 times in a row (28 success / 0 failed / 0 errors); fmt/lint/test all pass.

Changes

• e2c6a0f : test(persist): stabilize flaky session write fixture on nightly CI
  • Replace write_and_wait with a blocking store.write_sync call, removing the vim.wait-based async completion polling that introduced fixture-write nondeterminism.
  • Document the rationale in a comment (async path still covered by other tests; root cause of the nightly flakiness).