fix(dynamo-gen): use robust DateTime check in UnixTimestampTypeHandler

[Im5tu]

Context

Follow-up to #74. That PR moved AttributeHandlerRegistry to a robust, symbol-based DateTime/DateTimeOffset check, but UnixTimestampTypeHandler.CanHandle was left on the brittle name-only comparison:

underlyingType.Name == nameof(DateTime) || underlyingType.Name == nameof(DateTimeOffset)

A user-defined type named DateTime (or DateTimeOffset) outside the System namespace would be mistaken for the BCL type.

Change

• Extract the robust check into a shared ITypeSymbol.IsDateTimeOrDateTimeOffset() extension (SpecialType.System_DateTime for DateTime; type name + System namespace via ToDisplayString() for DateTimeOffset, which has no SpecialType).
• Use it in both UnixTimestampTypeHandler.CanHandle and AttributeHandlerRegistry — single source of truth, removes the duplicated namespace logic.

Scope notes

Deliberately left untouched (out of scope of this finding):

• UnixTimestampTypeHandler.GenerateFromDynamoRecord name checks — they run only after CanHandle already confirmed the type, so they select a branch rather than gate.
• DateTimeTypeHandler / UnixTimestampAttributeHandler name-only checks — pre-existing, unrelated to this finding. The new extension is available to tighten them later.

Validation

• dotnet build of the generator: clean (0 warnings/errors).
• dotnet test: 359 passed, 0 failed — including a new regression test that builds a type named DateTime outside System and asserts CanHandle returns false (would have returned true under the old check).

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 3de00abb-77d6-4c71-87a7-91b59ea02c3f

📥 Commits

Reviewing files that changed from the base of the PR and between ce8c084 and ce48d53.

📒 Files selected for processing (4)

• src/Clients/Goa.Clients.Dynamo.Generator/Attributes/AttributeHandlerRegistry.cs
• src/Clients/Goa.Clients.Dynamo.Generator/Models/TypeSymbolExtensions.cs
• src/Clients/Goa.Clients.Dynamo.Generator/TypeHandlers/UnixTimestampTypeHandler.cs
• tests/Clients/Goa.Clients.Dynamo.Generator.Tests/TypeHandlers/UnixTimestampTypeHandlerTests.cs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/dynamo-unixtimestamp-handler-check

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