fix: fix: restore missing WebChat sessions

[Alchuang22-dev]

Fix #9605 issue

Modifications / 改动点

修复了后端消息入口直接写入 platform_message_history，没有查询或补建 platform_sessions导致的 sidebar 相关问题。对应代码在chat_service.py (line 1090)

Fix WebChat conversations that have message history but are missing from the sidebar because no corresponding platform_sessions record exists.

• Ensure a WebChat platform session exists when processing a message.

• Make the existing WebChat session migration run idempotently on every startup.

• Preserve normal message delivery if session backfilling fails.

• Add a focused regression test based on the existing ChatService test fixture.

• 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

在 test_chat_route.py 中添加了相关的新测试。

Commands：

• ruff format .
• ruff check .
• pytest tests/test_chat_route.py -q — 10 passed

1. 生成一个对话

2. 人为复现issue里的历史

3. 重新查询，修复后的预期结果是 1

4. UI也正常

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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 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。

PTAL!

Summary by Sourcery

Ensure WebChat conversations are correctly surfaced by creating missing platform sessions at message processing time and making the WebChat session migration rerunnable.

Bug Fixes:

• Restore WebChat conversations that had message history but were missing from the sidebar due to absent platform session records.
• Prevent failures in WebChat session backfilling from disrupting normal message delivery.

Enhancements:

• Make the WebChat platform session migration idempotent so it can safely run on every startup.

Tests:

• Add an async regression test verifying that processing a WebChat message creates a missing platform_session entry.

[sourcery-ai]

Hey - I've left some high level feedback:

• In test_chat_stream_creates_missing_webchat_platform_session, the teardown uses chat_service.webchat_queue_mgr.remove_queues(session_id) instead of service.webchat_queue_mgr.remove_queues(session_id), which will raise a NameError and should be corrected.
• The new WebChat session backfill in build_chat_stream relies on the literal string platform_history_id == "webchat"; consider centralizing this identifier (or aligning with the platform id constants already used elsewhere) to avoid subtle mismatches and make future changes easier.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In `test_chat_stream_creates_missing_webchat_platform_session`, the teardown uses `chat_service.webchat_queue_mgr.remove_queues(session_id)` instead of `service.webchat_queue_mgr.remove_queues(session_id)`, which will raise a NameError and should be corrected.
- The new WebChat session backfill in `build_chat_stream` relies on the literal string `platform_history_id == "webchat"`; consider centralizing this identifier (or aligning with the platform id constants already used elsewhere) to avoid subtle mismatches and make future changes easier.

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