fix: parse cursor pagination with json.loads on resume

[anaselmhamdi]

Summary

On resume, the producer crashed with:

ValueError: malformed node or string on line 1: <ast.Name object at 0x...>
  File ".../bizon/engine/pipeline/producer.py", line 61, in get_or_create_cursor
    pagination=ast.literal_eval(cursor_from_db.pagination),

Root cause: pagination is written as JSON (json.dumps at backend/adapters/sqlalchemy/backend.py:296 and :398), but read back with ast.literal_eval. ast.literal_eval parses Python literal syntax; JSON's true / false / null become ast.Name nodes and raise. Any pagination dict containing a bool or None — e.g. {"has_more": true, "cursor": null} — was unrecoverable. The pipeline exited with BACKEND_ERROR on every retry.

Impact: Connectors that currently break on resume

• Notion — has_more, data_sources_loaded: True, next_cursor can be None
• Cycle — passes through GraphQL pageInfo which includes hasNextPage (bool)

Other connectors (Dummy, GSheets, Kafka, PokeAPI, SanaAI, Periscope) only store strings/lists/ints and worked by accident.

Change

• bizon/engine/pipeline/producer.py — swap ast.literal_eval → json.loads, matching the write path. No migration needed: json.dumps has been the writer since the initial commit.
• tests/engine/test_producer_recovery.py — new regression test test_recovery_with_json_only_pagination_values persisting {"cursor": ..., "has_more": True, "next_page": None} and asserting recovery. Fails on main with the exact bug-report stacktrace.

Test plan

• uv run pytest tests/engine/test_producer_recovery.py -v — both tests pass
• uv run pytest tests/engine/ --ignore=tests/engine/test_parse_yaml.py --ignore=tests/engine/queue/test_queue_factory.py — 34 passed; remaining failures are unrelated missing-extras (postgres/bigquery/kafka)
• make format — clean
• Verify a Notion or Cycle pipeline resumes cleanly in staging after this ships

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