Fix AutoReversingMigration for Alter.Table().AddColumn()

[Copilot]

AutoReversingMigration throws NotSupportedException when reversing migrations that use Alter.Table().AddColumn(). This worked in 3.3.2 but regressed in 7.1.

// This migration fails to reverse
public class AddColumnsToLibrary : AutoReversingMigration
{
    public override void Up()
    {
        Alter.Table("Library")
            .AddColumn("details_ar").AsString(2000).Nullable()
            .AddColumn("address_ar").AsString(250).Nullable();
    }
}

Root cause: Alter.Table() adds an AlterTableExpression to the context (holding table metadata), then each .AddColumn() adds a CreateColumnExpression. The CreateColumnExpression.Reverse() works correctly, but AlterTableExpression inherited the base class Reverse() which throws.

Changes:

• Add Reverse() override to AlterTableExpression that returns a copy of itself (metadata-only expression, no reversal semantics needed)
• Add unit test reproducing the exact issue scenario
• Add unit test for AlterTableExpression.Reverse() directly

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>AutoReversingMigration on PostgreSQL used to work in 3.3.2 and .NET 8. It stops working on 7.1 and .NET 10.</issue_title>
<issue_description>Describe the bug

[Migration(85)]
public class _0085_AddArabicColumnsToLibraryTable : AutoReversingMigration
{
    public override void Up()
    {
        Alter.Table(Tables.Library)
            .AddColumn("organization_details_ar").AsString(StringLength.TwoThousand).Nullable()
            .AddColumn("details_ar").AsString(StringLength.TwoThousand).Nullable()
            .AddColumn("address_ar").AsString(StringLength.TwoHundredFifty).Nullable();
    }
}

Expected behavior
Reverse migration works

Information (please complete the following information):
.NET 10

    <PackageReference Include="FluentMigrator" Version="7.1.0" />
    <PackageReference Include="FluentMigrator.Runner" Version="7.1.0" />
    <PackageReference Include="FluentMigrator.Runner.Postgres" Version="7.1.0" />
    <PackageReference Include="Npgsql" Version="10.0.0" />
    <PackageReference Include="Npgsql.NetTopologySuite" Version="10.0.0" />

Error

87: _0087_AlterAddIsAffiliateToPersonnelTable reverting
-------------------------------------------------------------------------------
Beginning Transaction
Rolling back transaction
Unhandled exception: System.NotSupportedException: The AlterTableExpression cannot be automatically reversed
   at FluentMigrator.Expressions.MigrationExpressionBase.Reverse()
   at FluentMigrator.AutoReversingMigration.<>c.<GetDownExpressions>b__1_0(IMigrationExpression e)
   at System.Linq.Enumerable.ListSelectIterator`2.Fill(ReadOnlySpan`1 source, Span`1 destination, Func`2 func)
   at System.Linq.Enumerable.ListSelectIterator`2.ToList()
   at System.Linq.Enumerable.ReverseIterator`1.ToList()
   at FluentMigrator.AutoReversingMigration.GetDownExpressions(IMigrationContext context)
   at FluentMigrator.Runner.MigrationRunner.<>c.<ApplyMigrationDown>b__67_0(IMigration m, IMigrationContext c)
   at FluentMigrator.Runner.MigrationRunner.ExecuteMigration(IMigration migration, Action`2 getExpressions)
   at FluentMigrator.Runner.MigrationRunner.ApplyMigrationDown(IMigrationInfo migrationInfo, Boolean useTransaction)
   at FluentMigrator.Runner.MigrationRunner.MigrateDown(Int64 targetVersion, Boolean useAutomaticTransactionManagement)
   at FluentMigrator.Runner.MigrationRunner.MigrateDown(Int64 targetVersion)
   at Program.<<Main>$>g__RollbackDatabase|0_4(IServiceProvider serviceProvider, Int64 rollbackVersion) in D:\GitHub\bias\src\Data\BIAS.Migration\Program.cs:line 105
   at Program.<>c__DisplayClass0_0.<<Main>$>b__1(ParseResult parseResult) in D:\GitHub\bias\src\Data\BIAS.Migration\Program.cs:line 56
   at System.CommandLine.Command.<>c__DisplayClass30_0.<SetAction>b__0(ParseResult context)
   at System.CommandLine.Invocation.AnonymousSynchronousCommandLineAction.Invoke(ParseResult parseResult)
   at System.CommandLine.Invocation.InvocationPipeline.InvokeAsync(ParseResult parseResult, CancellationToken cancellationToken)
```</issue_description>

<agent_instructions>Also add new tests with a similar use case</agent_instructions>

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

<comments>
</comments>

• Fixes AutoReversingMigration on PostgreSQL used to work in 3.3.2 and .NET 8. It stops working on 7.1 and .NET 10. #2195

---

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Test Results

    2 files  ±0      2 suites  ±0   38s ⏱️ -1s
5 355 tests ±0  5 023 ✅ +1  332 💤 ±0  0 ❌  - 1 
5 836 runs  ±0  5 240 ✅ +1  596 💤 ±0  0 ❌  - 1 

Results for commit 62c1b28. ± Comparison against base commit fcc42a9.

♻️ This comment has been updated with latest results.