Follow-up to #23220, addressing Cian's review comments:
- **SQL casing**: Uppercase `UNNEST` to match `NULLIF`/`COALESCE`
convention in the query.
- **Builder pattern**: `chatMessage` struct now uses unexported fields
with a `newChatMessage` constructor for required fields (role, content,
visibility, modelConfigID, contentVersion) and chainable builder methods
(`withCreatedBy`, `withCompressed`, `withUsage`, `withContextLimit`,
`withTotalCostMicros`, `withRuntimeMs`) for optional/nullable fields.
- **Batch test in chats_test**: Replaced the `for i := 0; i < 2` loop
with a single batch insert of 2 messages to actually exercise the batch
logic.
- **Multi-message querier test**: Added `BatchInsertMultipleMessages`
test verifying 3-message batch insert with role ordering, sequential
IDs, nullable field semantics (NULL for zero UUIDs and zero ints), and
token/cost assertions.
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Co-authored-by: Cian Johnston <cian@coder.com>
Kyle Carberry
d4a072b61e