Make BulkOperations provider-agnostic beyond SQL Server

[cursor]

Problem

BulkOperations<TEntity> is documented as supporting batched INSERT for non-SQL Server providers, but the implementation directly imports Microsoft.Data.SqlClient and uses SqlBulkCopy for SQL Server.

File: DBTools/Bulk/BulkOperations.cs

Non-SQL Server bulk paths may be incomplete or untested.

Proposed solution

1. Abstract bulk insert behind IDbProvider or a dedicated IBulkInsert interface
2. Keep SqlBulkCopy as SQL Server optimization only
3. Implement/test batched INSERT fallback for PostgreSQL, MySQL, SQLite
4. Add tests (see Add unit tests for async, context, bulk, interceptors, and mapping #28)

Acceptance criteria

• No direct SqlConnection/SqlBulkCopy usage outside SQL Server code path
• Bulk insert works (or fails clearly) per provider
• Provider-specific behavior documented in README/API reference

Related

#28 (tests), #32 (multi-provider migration)