SDK: propagate server and statement failures through public write APIs #164

Problem

Python and JavaScript convert a server error into a one-row result, allowing public inserts to report success after a failed durable write. Structured statement errors and chunked results also need one consistent failure contract.

Evidence

Source: 5c1f313379a5cb2995a43a66ddba0b5cc9798b7f (inspected 2026-10-03). These are source findings; the acceptance scenarios below still need implementation and execution.

• packages/inputlayer-py/src/inputlayer/connection.py:265: ErrorResponse is converted to ResultResponse with an error row.
• packages/inputlayer-js/src/connection.ts:240: The JavaScript error path constructs a successful-shaped row response.
• packages/inputlayer-py/src/inputlayer/connection.py:300: Chunk assembly reconstructs the result and must preserve statement-failure metadata.

Acceptance criteria

• Raise typed server/statement errors from public insert, load and migration APIs; preserve original code, message and statement index across chunk assembly.
• Test real-shaped server errors and partial result.errors through both public SDKs, then a real engine failed insert. No success count may be returned after failure.
• Keep healthy-response checks local without an added round trip and pass Performance: baseline and mandatory pre-PR regression pipeline #163. The original storage-loss issue Engine: fact inserts return success but store nothing when the data directory vanishes (WAL write failure swallowed) #93 also needs its separate R10 health/recovery probe before closure.

Delivery

Plan item: S02. R1 - Safety + perf/e2e gates.

Dependencies / related work: #163; contributes #93.

Contributes to #93; completing this slice alone does not close the broader issue.