Add explicit tests for UseNpgsql() without connection string

[Copilot]

The issue claimed UseNpgsql() fails without a connection string, unlike SQL Server/SQLite providers. This is incorrect - the parameterless overload has existed since initial implementation (lines 28-41 of NpgsqlDbContextOptionsBuilderExtensions.cs).

Changes

• Added Can_add_extension_without_connection_string() test
• Added Can_add_extension_without_connection_string_using_generic_options() test

Both verify that UseNpgsql() can be called without parameters and that ConnectionString, Connection, and DataSource are all null until explicitly set.

Usage

public class MyDbContextFactory : IDesignTimeDbContextFactory<MyDbContext>
{
    public MyDbContext CreateDbContext(string[] args)
    {
        var optionsBuilder = new DbContextOptionsBuilder<MyDbContext>();
        optionsBuilder.UseNpgsql(); // Valid - connection provided later via DI or runtime
        return new MyDbContext(optionsBuilder.Options);
    }
}

The error "The ConnectionString property has not been initialized" occurs at connection time, not configuration time. This is expected - a connection must be provided before use via DI (AddNpgsqlDataSource), SetConnectionString(), or SetDbDataSource().

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

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• www.myget.org
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If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
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Original prompt

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This section details on the original issue you should resolve

<issue_title>IDesignTimeDbContextFactory without a connection string</issue_title>
<issue_description>In other providers such as Microsoft.EntityFrameworkCore.Sqlite or Pomelo.EntityFrameworkCore.MySql, you can implement it by calling UseSqlite() or UseMySql() without parameters.

That is, without connecting to a real database.

With Npgsql.EntityFrameworkCore.PostgreSQL and UseNpgsql(), I get the following error:

Unable to create a 'DbContext' of type 'MyDbContext'. The exception 'The ConnectionString property has not been initialized.' was thrown while attempting to create an instance. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728

Code:

using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Design;

public class MybContextContextFactory : IDesignTimeDbContextFactory<MyDbContext>
{
    public MyDbContext CreateDbContext(string[] args)
    {
        DbContextOptionsBuilder<MyDbContext> dbContextOptionsBuilder = new();
        dbContextOptionsBuilder.UseNpgsql();

        return new MyDbContext(dbContextOptionsBuilder.Options);
    }
}

Command:

dotnet ef dbcontext script --output Databas.sql

cc @roji</issue_description>

<agent_instructions>Verify that what the user is asserting is true (that it's impossible to call UseNpgsql() without a connection string. If so, compare the PG provider in this repo with the SQL Server provider implemenation in https://github.com/dotnet/efcore/issues and align the PG provider with the SQL Server pattern to allow it. Port over relevant tests as well.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes IDesignTimeDbContextFactory without a connection string #3706

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