feat(session): 返回 memory diff URI

[qin-ctx]

Description

本 PR 调整 session commit 的 memory_diff_uri 暴露时机：POST /sessions/{id}/commit 的即时响应仍只表示任务已 accepted，不返回 memory_diff_uri；后台 Phase 2 中长记忆抽取完成后，会先检查 memory_diff.json 是否实际存在，存在时才在 task result 返回 memory_diff_uri，方便调用方确认本次归档对应的记忆 diff 文件。

长记忆抽取阶段如果没有生成任何 memory operations，也会由 SessionCompressorV2 写入空结构的 memory_diff.json，避免“长记忆抽取已完成但没有 diff 文件”的状态。

示例：

{
  "status": "accepted",
  "task_id": "uuid-xxx",
  "archive_uri": "viking://user/alice/sessions/a1b2c3d4/history/archive_001",
  "archived": true
}

任务完成后：

{
  "status": "completed",
  "result": {
    "session_id": "a1b2c3d4",
    "archive_uri": "viking://user/alice/sessions/a1b2c3d4/history/archive_001",
    "memory_diff_uri": "viking://user/alice/sessions/a1b2c3d4/history/archive_001/memory_diff.json",
    "memories_extracted": {
      "memory_write": 1
    }
  }
}

Related Issue

无

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• 调整 session.commit 即时响应：不再提前返回 memory_diff_uri。
• 在长记忆抽取成功完成后，基于 VikingFS.exists() 确认 memory_diff.json 存在，再将 memory_diff_uri 写入 task result；execution-only、跳过长记忆抽取或文件未产出时不返回该字段。
• 将无 operations 的空 memory_diff.json 生成收敛到 SessionCompressorV2.extract_long_term_memories 内部，并同步中英文文档与测试。

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

已运行：

• python -m py_compile openviking/session/session.py openviking/session/compressor_v2.py
• git diff --check -- openviking/session/session.py openviking/session/compressor_v2.py tests/session/test_session_commit.py tests/server/test_api_sessions.py docs/en/api/05-sessions.md docs/zh/api/05-sessions.md
• env UV_CACHE_DIR=/private/tmp/openviking-uv-cache uv run python -m pytest tests/server/test_api_sessions.py::test_compress_session

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

不适用

Additional Notes

memory_diff_uri 现在表示“长记忆抽取阶段已完成，并且对应 diff 文件已经实际存在”。该字段不会出现在 commit accepted 响应中，也不会在没有调度长记忆抽取或未产出 diff 文件的 task result 中返回。