generics: populate WithResult error

[qqxhb]

Fix mismatch with gorm.io Create docs generics example: WithResult() now exposes Error and it is populated when statement execution fails.

Changes:

• Add Error field to WithResult result and reset it per statement.
• Populate result.Error via DB.AddError when a WithResult result is attached.
• Add sqlite-based unit test covering success + failure cases.

Tests:

• go test ./...

[propel-code-bot]

This update specifically aligns WithResult behavior for Create statements so execution errors are surfaced when a result is attached.

Affected Areas

• generics.go
• gorm.go
• generics_withresult_test.go

This summary was automatically generated by @propel-code-bot