fix(offchain): warn on missing 'result' key in ok envelope; test coverage

[DIvyaNautiyal07]

Summary

Two follow-ups to review nits on #98.

1. Missing result key silently yielded None

If the mech's response schema drifts (renamed field, truncated payload, added nesting), envelope.get("result") returned None, _PollSnapshot was constructed with result=None, and _apply_snapshot set target.result=None, target.error="Unknown". That is indistinguishable from a routine tool failure and lands in mech_responses as a fake failed prediction.

Now logs a warning citing the observed envelope keys so the drift is visible. Every other structural failure in _parse either returns processing or emits a warning; this one now matches the convention.

2. Restored _serialise_result JSON-coercion coverage

The removed test used {"answer": 1} (dict) as the response value, exercising the non-string fallback (json.dumps). The two new tests on #98 feed string results, so nothing pinned the coercion branch.

Added:

• test_ok_status_serialises_non_string_inner_result_to_json — feeds result={"p_yes": 0.6, "p_no": 0.4} and asserts the snapshot result is the JSON-encoded string.
• test_ok_status_envelope_missing_result_key_warns — asserts the schema-drift warning fires. Stub logger now captures warnings into a list so tests can assert on them.

Verification

• 14 tests pass in test_offchain_response_poller.py (12 pre-existing + 2 new)
• tomte tox -p -e black-check -e isort-check -e flake8 -e mypy -e pylint -e darglint all green
• autonomy packages lock --check green

Follow-up to

Review comments on #98 (already merged into feat/offchain-epic).

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