[release/10.0] Fix NullReferenceException when scaffolding named default constraints

[Copilot]

Fixes #37175

Description
When scaffolding from a database with dotnet ef dbcontext scaffold, a NullReferenceException occurs in SqlServerAnnotationCodeGenerator.GenerateFluentApiCalls for columns that have both a named default constraint (e.g., DF_TableName_ColumnName) and a default value equal to the CLR default (e.g., DEFAULT ((0)) for int).

The SqlServerAnnotationCodeGenerator.GenerateFluentApiCalls method assumed that if DefaultConstraintName exists, either DefaultValue or DefaultValueSql must also exist. However, the base scaffolding logic in ScaffoldingModelExtensions.GenerateAnnotations can remove these annotations when the default value equals the CLR default, leaving only DefaultConstraintName.

Customer impact
Scaffolding fails completely when the database contains tables with named default constraints where the default value equals the CLR default (e.g., DEFAULT ((0)) for int). No workaround exists other than removing the named default constraint from the database or avoiding scaffolding those tables.

How found
Customer reported on EF 10

Regression
Yes. This is a regression from EF Core 9.0, introduced by PR #36067 as previously the constraint name was just ignored. #36567 attempted to fix a similar scenario but didn't account for this case.

Testing
Test added

Risk
Low. The fix simply skips generating fluent API code for the constraint name instead of throwing.

Original prompt

Fix #37175. Start by creating a test that fails with the same or similar exception

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[AndriySvyryd]

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[AndriySvyryd]

cc @artl93