feat: added GET /sessions endpoint with pagination

[hk2166]

Implements GET /sessions to list all active server-side sessions with basic
metadata (session ID, message count, last accessed timestamp) and pagination support.

Fixes #218. Complements #196 (GET message history) — together they allow complete
session recovery after a browser restart or sessionStorage clear.

Changes:

• api/models/schemas.py — Added SessionInfo and SessionListResponse Pydantic models
• api/services/memory.py — Added list_sessions(page, page_size) using a single
  lock acquisition to ensure a consistent snapshot of total count and session metadata
• api/routes/chatbot.py — Registered GET /sessions with page (default 1) and
  page_size (default 20, clamped to 1–100) query parameters
• tests/unit/mocks/test_env.py — Added mock_list_sessions fixture
• tests/unit/routes/test_chatbot.py — 6 new unit tests
• tests/integration/test_chatbot.py — 7 new integration tests

Testing done

Automated tests were added at both unit and integration level. All 33 tests pass:

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira — Closes Add GET /sessions endpoint to list all active sessions #218
• Link to relevant pull requests, esp. upstream and downstream changes — relates to feat: add GET /sessions/{session_id}/message endpoint #196, feat: add GET /sessions endpoint with pagination #238
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[hk2166]

Adds unit tests for BM25Indexer in rag/embedding/bm25_indexer.py which had zero test coverage. Closes #290

Testing done

12 unit tests added, all passing (pytest tests/unit/rag/embedding/test_bm25_indexer.py -v — 12 passed in 0.02s), covering build(), _index_config(), and get() across both available and unavailable SparseRetriever scenarios.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira — Closes #290
• Link to relevant pull requests, esp. upstream and downstream changes — N/A
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[berviantoleo]

Duplicates.

[berviantoleo]

#238

[berviantoleo]

Before submitting please look at the openned PR and give feedback in that PR.

This PR looks like combining two issues, it makes harder to review.