fix: preserve trace snapshot ordering and stabilize cross-platform tests

[wcqqq1214]

Fix the restart-test hang introduced after #10614 and the trace ordering and Windows test failures exposed afterward.

Modifications / 改动点

• Mock and verify asynchronous trace cleanup in the restart test.

• Keep snapshot timestamps increasing within each trace so older updates cannot overwrite completion when the clock repeats or moves backward. Span start/end times are unchanged.

• Use short pytest parameter IDs to avoid Windows environment-variable length limits.

• Move the dashboard test’s flush/cleanup off the event loop; allow a 30-second flush timeout with failure diagnostics.

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

• Focused regression suite: 256 passed, including repeated/backward clocks and reversed snapshot order; the clock regressions failed before the fix.
• ruff format ., ruff check ., and git diff --check passed.
• All 23 CI checks passed for 02539ce38. Windows: 4622 passed, 62 skipped, no failures or errors. CI results.
• The Windows persistence timeout was not reproduced locally; the adjusted wait passed CI.

No WebUI or API schema changes; screenshots and documentation updates are not applicable.

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Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
  / 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。

• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
  / 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。

• 📚 I checked the affected WebUI instructions and screenshots in docs/zh and docs/en against the changed navigation, page structure, and labels, and updated them in this PR (or explained why no documentation update is needed). For renamed, moved, or merged entry points, I included an old entry → new entry mapping in the docs and changelog.
  / 我已对照变化后的 WebUI 入口、页面结构和术语，核对并在本 PR 中更新 docs/zh 和 docs/en 的相关操作说明与截图（或说明无需更新文档的原因）。入口改名、移动或合并时，已在文档和 changelog 中补充 旧入口 → 新入口 对照。

• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
  / 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到 requirements.txt 和 pyproject.toml 文件相应位置。

• 😮 My changes do not introduce malicious code.
  / 我的更改没有引入恶意代码。

Summary by Sourcery

Stabilize trace persistence ordering and cross-platform lifecycle tests.

Bug Fixes:

• Preserve trace snapshot ordering so completed traces cannot be reopened by repeated or regressing wall-clock timestamps.
• Prevent restart and dashboard persistence tests from hanging or timing out during threaded cleanup and flushing.
• Stabilize long-prompt parameterization across Windows test environments.

Tests:

• Expand trace-store regression coverage for real, frozen, backward-moving, and reversed snapshot timestamps.
• Add assertions that restart cleanup is dispatched and awaited correctly, with bounded persistence flushing and diagnostics.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Approved.

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Sourcery withdrew this approval because the latest commits introduced blocking findings.

[wcqqq1214]

@sourcery-ai review

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Needs a human reviewer. If the timestamp logic is wrong, persisted trace snapshots can retain incorrect ordering or timestamps after the code is reverted; those historical values are not automatically repaired. The impact is bounded to trace data and can be addressed by clearing or correcting affected records.

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