Allow customizing logger for SQL errors

[dknecht]

Problem

The sql method in Agent uses console.error directly for logging SQL errors:

} catch (e) {
  console.error(`failed to execute sql query: ${query}`, e);
  throw this.onError(e);
}

This bypasses any structured logging the application may have configured. During DO resets (e.g., code deployments), this produces unstructured logs like:

(error) failed to execute sql query:
    INSERT OR REPLACE INTO cf_agents_state (id, state)
    VALUES (?, ?)
   Error: Durable Object reset because its code was updated.

Proposal

Allow users to customize logging behavior, either via:

1. Override method - onSqlError(query: string, error: unknown) that defaults to console.error but can be overridden
2. Logger injection - Accept a logger in Agent options/constructor
3. Use observability - Route through the existing observability interface

Option 1 is minimal and consistent with existing patterns like onError.