Analyst tools report backend failures as success, refuse valid read-only SQL, and clip long text with no way to read the rest #1447

Observed while running the read-only analyst against real warehouses:

• Tool results carrying a backend error (metadata.error, success:false, explicit error envelope) were returned with status: ok, so the model, result lint and telemetry treated failures as successes. 64 of 98 observed execution errors in one set of sessions were reported this way (51 backend errors, 13 Python tracebacks from bash).
• The read-only SQL classifier refused valid read-only syntax as writes or unsupported: DuckDB integer division //, NOT GLOB on SQLite/DuckDB, DuckDB qualified prior.* aliases, standalone or CTE PIVOT/UNPIVOT.
• Mongo commands written as {collection, find|aggregate|count|distinct} were rejected with command "undefined" is not allowed.
• JSONL spill rows could omit absent values, producing ragged rows.
• read clips lines at 2,000 characters with no way to read the remainder; sql_execute previews clip long text cells with no way to fetch a complete cell, so a long policy document could never be read in full.