[Tejas Lamba (TejasLamba2006)]

Summary

When the proposer returns wrongly-typed fields, parse_payload crashes instead of filtering them out, causing the session to stay pending and retry the proposer on every session start.

Changes

• obj.get("extractors", []) returns None (not []) when the key exists with value null. Changed to obj.get("extractors") or []
• Filter extractors with non-string pattern field
• Filter facts with non-dict detector field
• Guard _rx() against non-string input

Test plan

Inline verification:

parse_payload('{"extractors": null, "facts": []}')  # was TypeError, now ([], [])
parse_payload('{"extractors": [{"name": "x", "pattern": [1,2]}], "facts": []}')  # filters out
parse_payload('{"extractors": [], "facts": [{"id": "f1", "detector": "string"}]}')  # filters out
_rx(['a', 'b'])  # was TypeError, now None

Fixes #173