fix: harden session restore against malformed persisted JSON (fixes #304)

[Flamki]

Fixes #304.

This change makes session restore more resilient when a persisted session file is corrupted.

What was happening

load_session() read JSON directly from disk. If one session file was malformed (for example truncated JSON), restore could fail on that file and affect startup/session reload flow.

What this PR changes

• In chatbot-core/api/services/sessionmanager.py:
  • Guarded JSON load in _load_session_from_json().
  • If file read fails (OSError) or JSON is invalid (JSONDecodeError), we now log a warning and return [].
  • If payload shape is not a list, we log a warning and return [].
• In chatbot-core/tests/unit/services/test_sessionmanager.py:
  • Added tests for malformed JSON fallback.
  • Added tests for non-list payload fallback.
  • Added test to ensure reload_persisted_sessions() restores valid files and skips malformed ones.

Scope

Session persistence robustness only. No change to reply generation or endpoint contracts.

Verification

• pytest chatbot-core/tests/unit/services/test_sessionmanager.py -q
• pytest chatbot-core/tests/unit/services/test_memory.py -q
• pylint chatbot-core/api/services/sessionmanager.py chatbot-core/tests/unit/services/test_sessionmanager.py