fix: annotate @default:null members as nullable

[Alan4506]

Problem

The Secrets Manager and Cognito Identity models have structure members with smithy.api#default": null. For example, in secrets-manager.json:

• DeleteSecretRequest$RecoveryWindowInDays (target long), @default: null at line 614.
• DeleteSecretRequest$ForceDeleteWithoutRecovery (target boolean), @default: null at line 621.

StructureGenerator.writeProperties only added the | None annotation for members with no @default trait, so these members resolved their default to None but kept the bare target annotation:

recovery_window_in_days: int = None
force_delete_without_recovery: bool = None

None is not assignable to int / bool, so pyright (run by PythonFormatter during codegen) fails the build:

src/aws_sdk_secretsmanager/models.py - error: Type "None" is not assignable to declared type "int" (reportAssignmentType)
src/aws_sdk_secretsmanager/models.py - error: Type "None" is not assignable to declared type "bool" (reportAssignmentType)

The fix

In the optional-members loop of StructureGenerator.writeProperties, mark the field nullable when the resolved default is a null node, so it is annotated | None:

if (!target.isDocumentShape()
        && member.expectTrait(DefaultTrait.class).toNode().isNullNode()) {
    writer.putContext("nullable", true);
}

This produces:

recovery_window_in_days: int | None = None
force_delete_without_recovery: bool | None = None

Document shapes are excluded. A document with a null default is a non-None Document holding a null value, generated via default_factory as field(default_factory=lambda: Document(None)). Annotating it | None would be wrong.

Testing

• Regenerated Secrets Manager and Cognito Identity: both build cleanly, and pyright reports 0 errors on the generated source.
• Regenerated DynamoDB, SQS and all existing clients in aws-sdk-python repo (no @default: null members): generated source is unchanged.
• :core:build (unit tests and the weather :core:integ integration test, whose model includes a Document = null member) passes.

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