NoSQL update_by_conditions — update records matching search_by_conditions criteria

An alternative to SQL UPDATE that operates in the NoSQL `search_by_conditions` format. Operators can update a set of records matching specified conditions without writing SQL.

## Proposed API

```json
{
"operation": "update_by_conditions",
"database": "data",
"table": "orders",
"conditions": [
{ "attribute": "status", "type": "equals", "value": "pending" },
{ "attribute": "age", "type": "greater_than", "value": 7 }
],
"records": { "status": "expired" }
}
```

## Acceptance criteria

- Accepts the same `conditions` format as `search_by_conditions`.
- Applies the given attribute updates to all matching records (merge, not replace).
- Returns a count of updated records.
- Documented in the Ops API docs.

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Tracked in Jira: [CORE-2850](https://harperdb.atlassian.net/browse/CORE-2850)

🤖 Filed by Claude on behalf of Kris.