## Summary
When `agnes query --remote` references a column that doesn't exist on the FROM table, users were seeing `Table "<id>" must be qualified with a dataset` instead of the actually-useful `Unrecognized name: <column>` from BigQuery. Surface the first-attempt diagnostic now; keep the second-attempt context as `underlying_original`.
Reproduced against production:
```
$ agnes query --remote "SELECT COUNT(*) FROM unit_economics WHERE authorize_date = DATE '2025-05-06'"
Error: remote_estimate_failed (HTTP 400)
message: Could not estimate scan size for this query.
underlying: 400 ... Table "unit_economics" must be qualified with a dataset.
```
(`unit_economics` has `authorize_timestamp`, not `authorize_date`.)
## Test plan
- [x] New `test_remote_estimate_failed_surfaces_first_error_when_attempts_differ` asserts the first-attempt message wins, second-attempt is preserved as `underlying_original`, hint points to `agnes schema`.
- [x] Existing `test_guardrail_returns_400_remote_estimate_failed_on_double_parse_error` still passes (both attempts mocked to identical error).
- [x] `pytest tests/test_api_query_guardrail.py` clean.
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