fix(session): unify archive context abstracts

[qin-ctx]

Description

Update session context assembly so completed archive abstracts are exposed through a single pre_archive_abstracts list and the separate latest_archive_id field is removed.

Related Issue

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

• Remove latest_archive_id from session context responses and include the latest archive summary entry in pre_archive_abstracts.
• Update session context stats, CLI rendering, and the OpenClaw example plugin to consume the unified archive abstract list.
• Refresh API docs and affected tests to match the new response shape and archive index behavior.

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

Ran:

• .venv/bin/python -m pytest -q tests/server/test_api_sessions.py tests/server/test_http_client_sdk.py
• pnpm -C examples/openclaw-plugin test -- --run __tests__/context-engine-assemble.test.ts

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)

Additional Notes

• tests/session/test_session_context.py still hits an existing mock AGFS append/setup failure when run directly in the current environment, so it is not included in the passing local test claim above.

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