Add JSONPath edge-case tests to the connector test suite

[prat3ik]

Strengthen the connector tests with JSONPath edge cases.

Where: server/test_chatbot_connector.py (these run with plain python3, no deps).

Add cases for jsonpath_get:

• Bracketed indices: $.a[0].b and out-of-range indices return None.
• Missing keys return None (no exception).
• Non-string leaves (dict/list) handled by extract_reply_text (JSON-serialised).
• Empty / None path returns None.

Acceptance:

• New test_* functions pass via python3 server/test_chatbot_connector.py.